CLI: Add axel clean command

[jgrosso]

This should:

1. Remove the axelTemp directory

2. Remove hs files that correspond to Axel files. To avoid clobbering handwritten files (e.g. that, for whatever reason, accidentally have the same name), we should verify that the generated files were actually created by Axel.
   In fact, maybe we should include a checksum, in case the user made manual improvements (and in the case of mismatch, include a --force option or something to confirm). If we add this sort of header, maybe we could include Annotate transpiled files with the version of Axel used to generate them, and don't allow macro imports from incompatible versions #62 in its implementation.