Allow setting the TypeScript ES Target version

[brentlintner]

It may not be as big of an issue than with the JavaScript parser module, but it still should be settable.

Currently we are using the ESNext target.

ts.createSourceFile reference:

https://github.com/Microsoft/TypeScript/blob/master/src/compiler/parser.ts#L457

Types:

{
  ES3: 0,
  ES5: 1,
  ES2015: 2,
  ES2016: 3,
  ES2017: 4,
  ESNext: 5,
  Latest: 5
}