Add support for path aliases and more

[drgould]

This is a lot bigger than I had intended, I just kept finding little things to add. Anyway, here's what's changed:

• Add aliases config option to specify aliases for path resolution
• Support path aliases from webpack config
• Better wildcard detection using require.resolve
• Support "uncaptured" imports (e.g. import 'some/path';)
• Support specifying explicit extensions per wildcard import
• Use path.sep rather than hardcoded / for better cross-platform support

[vihanb]

Imports in the form of import * as A from './a'; seem to throw a/foo.js is not a directory error while failing to create a variable for the import?