Allow comments in tsconfig.json

It’s always nice to be able to document your configuration!tsconfig.json now accepts single and multi-line comments.

{
    "compilerOptions": {
        "target": "ES2015", // running on node v5, yaay!
        "sourceMap": true   // makes debugging easier
    },
    /*
     * Excluded files
      */
    "exclude": [
        "file.d.ts"
    ]
}