Autoclean

[simonask]

Add a command-line flag --autoclean which goes through all file in the output directory and checks if their path would match a build recipe. Then deletes the file. Then finally deletes .werk-cache.

Needed clarifications:

1. Should files listed in .werk-cache, but which don't match a build recipe (i.e. they come from a previous build) also be deleted?
2. Should depfiles be deleted as well? Doing so would require evaluating the build recipe bodies in addition to checking if the pattern is a match.