Implement allowJs, change diagnostics option

[blakeembrey]

Changing the method of getting diagnostics results in a huge speed-up. The old method was implemented for some reason or another (I think it was so all diagnostics would print or something) but I think the speed-up negates that. Here's the speed test before and after with typings-core.

npm run test-spec  28.59s user 0.93s system 92% cpu 31.925 total

npm run test-spec  8.50s user 0.65s system 72% cpu 12.548 total

Closes #123.

[coveralls]

Coverage decreased (-0.3%) to 87.907% when pulling 3a9a580 on allow-js into e988879 on master.