support getting contract from module; fix bug in which entire contract path was being replaced instead of just the beginning (if matching contracts in embark.json)

lib/core/config.js

@@ -146,8 +146,13 @@ Config.prototype.loadExternalContractsFiles = function() {
   let contracts = this.contractsConfig.contracts;
   for (let contractName in contracts) {
     let contract = contracts[contractName];
-    if (contract.file) {
+    if (!contract.file) {
+      continue;
+    }
+    if (fs.existsSync(contract.file)) {
       this.contractsFiles.push(new File({filename: contract.file, type: "dapp_file", basedir: '', path: contract.file}));
+    } else if (fs.existsSync(path.join('./node_modules/', contract.file))) {
+      this.contractsFiles.push(new File({filename: path.join('./node_modules/', contract.file), type: "dapp_file", basedir: '', path: path.join('./node_modules/', contract.file)}));
     }
   }
 };

lib/modules/solidity/index.js

@@ -22,7 +22,8 @@ class Solidity {
                      let filename = file.filename;
 
                      for (let directory of self.contractDirectories) {
-                       filename = filename.replace(directory, '');
+                       let match = new RegExp("^" + directory);
+                       filename = filename.replace(match, '');
                      }
 
                      file.content(function(fileContent) {

test_apps/test_app/config/contracts.json

@@ -63,6 +63,9 @@
           100
         ]
       },
+      "ERC20": {
+        "file": "zeppelin-solidity/contracts/token/ERC20/ERC20.sol"
+      },
       "SimpleStorageTest": {
         "file": "./some_folder/test_contract.sol",
         "args": [