Support Progressive Transpilation

[kasajian]

TypeScript Version:
1.7.5 / 1.8.0-beta / nightly (1.9.0-dev.20160217)

Expected behavior:
Would like a mechanism that allows one to progressively select which JavaScript features should be transpiled, and which do not need it. The concept is described here: https://medium.com/@kasajian/progressive-transpilation-def5da34282e#.l38ztasjl

This will allow the creation of custom builds for the specific targeted browser versions.

Actual behavior:

[DanielRosenwasser]

Thanks for filing, we're tracking granular feature compilation at #4692.