`build` should always remove all files first

[preaction]

A bunch of times while I'm fixing site problems, I'll accidentally have files in the build directory I didn't intend. Since the build directory isn't cleaned up ever, those files get deployed (or cause the deploy to fail).

[preaction]

This is only build, not the deploy. The deploy can choose to do what it does when it does it.

[preaction]

Build will always --clean (#71)