Add support for specifying custom env files in gradle (#52)

* Add support for specifying env files in gradle

* Update different environment selection for Android

README.md

@@ -82,7 +82,19 @@ By default react-native-config will read from `.env`, but you can change it when
 
 #### Android
 
-To pick which file to use in Android, just set `ENVFILE` before building/running your app. For instance:
+To pick which file to use in Android, set a variable in your `build.config` before the `apply from:`:
+
+```
+project.ext.envConfigFiles = [
+    debug: ".env.development",
+    release: ".env.production",
+    anyCustomBuildTypeName: ".env",
+]
+
+apply from: project(':react-native-config').projectDir.getPath() + "/dotenv.gradle"
+```
+
+Alternatively, you can set `ENVFILE` before building/running your app. For instance:
 
 ```
 $ ENVFILE=.env.staging react-native run-android

android/dotenv.gradle

@@ -1,6 +1,31 @@
+import java.util.regex.Matcher
+import java.util.regex.Pattern
+
+def getCurrentFlavor() {
+    Gradle gradle = getGradle()
+    String tskReqStr = gradle.getStartParameter().getTaskRequests()[0].args
+
+    Matcher matcher = Pattern.compile("([A-Z]\\w+)").matcher( tskReqStr )
+    if( matcher.find() ) {
+        return matcher.group(1).toLowerCase()
+    } else {
+        return "";
+    }
+}
+
 def readDotEnv = {
+    def envFile = ".env"
+
+    if (System.env['ENVFILE']) {
+      envFile = System.env['ENVFILE'];
+    } else if (project.hasProperty("envConfigFiles")) {
+      def possibleFile = project.envConfigFiles.get(getCurrentFlavor())
+      if (possibleFile) {
+        envFile = possibleFile;
+      }
+    }
+
     def env = [:]
-    def envFile = System.env['ENVFILE'] ?: ".env"
     println("Reading env from: $envFile")
     try {
       new File("$project.rootDir/../$envFile").eachLine { line ->