Revert "Use `replaceExtensions` option properly."

This reverts commit c50fb4a78f82ba813c85eef4df14d1bdfd11d5e3.

Apparently, I should have looked further, sorry for the wasted effort.

`this.extensions` is used by broccoli-filter internally and only calls
`processString` when the files extension matches.
Merge pull request #11 from rjackson/oops-im-dumb

Revert "Use `replaceExtensions` option properly."