gts indent doesn't match Google style

[gcc42]

Specifically when breaking lines. Google style has good line break rules that prettier's indent rules conflicts with. Given that gts enforces prettier/prettier, my fix gives me breaks like this:

new Error('This is a long string that we'll need to break. Google style'
    + 'recommends continuing at at least +4 spaces, while prettier enforces +2');

Here's the corresponding rule: https://google.github.io/styleguide/jsguide.html#formatting-indent

[bcoe]

@gcc42 I agree, we should make an effort to fix this.