use aliasify for aliasMappings

Files matched by aliasMappings might not be `require`d by the modules that browserify is building, so using browserify to `require` those matched files is slow and wasteful. This PR changes to behavior of aliasMappings to instead rewrite `require` statements that match the aliasMappings files to use the actual relative file paths.

Verified that unit-tests pass.

addresses #127