Parse configuration files

[anaisbetts]

This PR sets up methods to initialize a CompilerHost based on either a .babelrc file, or an electron-compile .compilerc which is similar to babelrc except instead of a list of options, it’s a list of MIME types and the options to set.

Without an environment:

{
  "text/javascript": {
    "presets": ["stage-0", "es2015", "react"],
    "sourceMaps": "inline"
  }
}

With environments:

{
  "env": {
    "production": {
      "text/javascript": {
        "presets": ["stage-0", "es2015", "react"],
        "sourceMaps": false
      }
    },
    "development": {
      "text/javascript": {
        "presets": ["stage-0", "es2015", "react"],
        "sourceMaps": "inline"
      }
    }
  }
}

/cc #31