Add support for passing more options to traceur

I was frustrated with not being able to transpile code that used the `let` and `const` bindings, so I added the ability to pass along the options hash to traceur. The configuration for transpiling code with block bindings becomes:

```javascript
    traceur: {
      options: {
        sourceMaps: true,
        blockBinding: true
      },
      release: {
        files: [
          { src: ['src/js/**/*.js'], dest: 'build/' }
        ]
      }
    }
```

and you should get support for all of the options defined here: https://github.com/google/traceur-compiler/blob/master/src/options.js#L246 although I can't find them documented outside of the code itself.