Fix various issues with packager editor launcher

Summary: There are a few small bugs with the code that launches the editor from the packager:

* First of all, the filepath is not escaped which means tokens like `(` or spaces will mess up the process execution. Dropbox unfortunately decided to use spaces in its enterprise product, so I was getting this error:

![screen shot 2015-07-11 at 3 20 54 pm](https://cloud.githubusercontent.com/assets/1135007/8635748/186e7f2e-27ea-11e5-8058-1f4dabb79634.png)

* Next, the line number argument formatting was assumed to be in a specific format (`:%d`) which actually errors out vim and other editors.
* Lastly, the process was started synchronously but not attached to the stdin / stdout of the parent process. This means that only editors like mvim, sublime, and others would work since they spawn a new window. Editors like emacs, vi, nano, etc wouldn't work and instead just hang at the command line.

So I whipped up this diff to fix a number of these issues, demo here:
http://recordit.co/M6zwiUj7hp

The demo shows both
Closes https://github.com/facebook/react-native/pull/1957

Reviewed By: @vjeux, @pcottle

Differential Revision: D2420941

Pulled By: @frantic

packager/launchEditor.js

@@ -10,7 +10,37 @@
 
 var chalk = require('chalk');
 var fs = require('fs');
-var exec = require('child_process').exec;
+var spawn = require('child_process').spawn;
+
+function isTerminalEditor(editor) {
+  switch (editor) {
+    case 'vim':
+    case 'emacs':
+    case 'nano':
+      return true;
+  }
+  return false;
+}
+
+function getArgumentsForLineNumber(editor, fileName, lineNumber) {
+  switch (editor) {
+    case 'vim':
+    case 'mvim':
+      return [fileName, '+' + lineNumber];
+    case 'atom':
+    case 'subl':
+    case 'sublime':
+      return [fileName + ':' + lineNumber];
+    case 'joe':
+    case 'emacs':
+      return ['+' + lineNumber, fileName];
+  }
+
+  // For all others, drop the lineNumber until we have
+  // a mapping above, since providing the lineNumber incorrectly
+  // can result in errors or confusing behavior.
+  return [fileName];
+}
 
 function printInstructions(title) {
   console.log([
@@ -25,28 +55,45 @@ function printInstructions(title) {
   ].join('\n'));
 }
 
+var _childProcess = null;
 function launchEditor(fileName, lineNumber) {
   if (!fs.existsSync(fileName)) {
     return;
   }
 
-  var argument = fileName;
-  if (lineNumber) {
-    argument += ':' + lineNumber;
+  var editor = process.env.REACT_EDITOR || process.env.EDITOR;
+  if (!editor) {
+    printInstructions('PRO TIP');
+    return;
   }
 
-  var editor = process.env.REACT_EDITOR || process.env.EDITOR;
-  if (editor) {
+  var args = [fileName];
+  if (lineNumber) {
+    args = getArgumentsForLineNumber(editor, fileName, lineNumber);
+  }
   console.log('Opening ' + chalk.underline(fileName) + ' with ' + chalk.bold(editor));
-    exec(editor + ' ' + argument, function(error) {
-      if (error) {
-        console.log(chalk.red(error.message));
-        printInstructions('How to fix');
+
+  if (_childProcess && isTerminalEditor(editor)) {
+    // There's an existing editor process already and it's attached
+    // to the terminal, so go kill it. Otherwise two separate editor
+    // instances attach to the stdin/stdout which gets confusing.
+    _childProcess.kill('SIGKILL');
+  }
+
+  _childProcess = spawn(editor, args, {stdio: 'inherit'});
+  _childProcess.on('exit', function(errorCode) {
+    _childProcess = null;
+
+    if (errorCode) {
+      console.log(chalk.red('Your editor exited with an error!'));
+      printInstructions('Keep these instructions in mind:');
     }
   });
-  } else {
-    printInstructions('PRO TIP');
-  }
+
+  _childProcess.on('error', function(error) {
+    console.log(chalk.red(error.message));
+    printInstructions('How to fix:');
+  })
 }
 
 module.exports = launchEditor;