Rework CLI interface to support non-directory-based usages.

Closes #6 and #7.
Abandon the no-source-directory, multiple-file usage pattern.

In practice this is never what one wants. Instead of doing

    bin/commonize file1.js file2.js file3.js output/

one should simply use the directory-first pattern:

    bin/commonize . output/ file1 file2 file3
Remove "ast-types" dependency.

Recast can provide all the functionality that ast-types provided.

Closes #2.
Bump NPM version to 0.6.3 in preparation for publishing.