Adds support for babel config file

[suchitadoshi1987]

• cleanup index.js - it uses the babel-utils functions and remove the duplicate functionalities
• default babel options are now the responsibility of the transpileTree
• cleaned up the code from getBabelOptions file where it should just return the plugin and preset config
• exported the helper methods from emberPlugins
• fix existing tests
• add tests for babelrc config
• add README for usage of babelConfig