AOT compile code if asked

[anaisbetts]

This PR allows you to pre-build the cache folder ahead of time for every file in your app, via a new electron-compile CLI app:

Usage: electron-compile --target [target-path] paths...

Options:
  -t, --target   The target directory to write a cache directory to
  -v, --verbose  Print verbose information
  -h, --help     Show help

This PR also adds new methods exported to the main object for compiling via code:

// Public: Compiles a single file given its path.
//
// filePath: The path on disk to the file
// compilers: (optional) - An {Array} of objects conforming to {CompileCache}
//                         that will be tried in-order to compile code. You must
//                         call init() first if this parameter is null.
//
// Returns a {String} with the compiled output, or will throw an {Error} 
// representing the compiler errors encountered.
export function compile(filePath, compilers=null)

// Public: Recursively compiles an entire directory of files.
//
// rootDirectory: The path on disk to the directory of files to compile.
// compilers: (optional) - An {Array} of objects conforming to {CompileCache}
//                         that will be tried in-order to compile code.
//
// Returns nothing.
export function compileAll(rootDirectory, compilers=null)

// Public: Allows you to create new instances of all compilers that are 
// supported by electron-compile and use them directly. Currently supports
// Babel, CoffeeScript, TypeScript, LESS, and Sass/SCSS.
//
// Returns an {Array} of {CompileCache} objects.
export function createAllCompilers()

One Last Thing

This PR also fixes a semi-related bug where require in the renderer process wouldn't get transpiled files.