[RFC] Add support for namespace imports

[eventualbuddha]

cc @caridy @ericf @wycats @dherman

This adds support for namespace imports, i.e. import * as foo from 'foo'. The initial work was done by @caridy and I followed up to make it work with both the commonjs and bundle formatters. I am not yet satisfied with this branch. The biggest code smell is that I am having to cache the specifier names by forcing them to be computed before they are re-written. Other than that, here's some things I think we may need before we can merge:

• [x] Make the tests added in 56aa11e pass.
• [ ] Potentially optimize import * as foo from 'foo' with foo.a to become a direct reference to the a export (i.e. foo$$a) rather than foo$$.a, which may keep us from needing the foo$$ object at all (bundle formatter only).
• [ ] Figure out a better way to preserve the original specifier names.
• [x] Ensure ES5 output is acceptable for the bundle formatter (i.e. get properties vs. Object.defineProperty).

Any other optimizations or static errors you all can think of?

[caridy]

it looks good to me, let me know when you merge it to try it out with the other formatters.

[eventualbuddha]

Released in v0.9.0.