Add anaphoric if statement
Add ap-if docs.
Only call macroexpand when needed

We only need to call macroexpand on HyExpressions,
as this is already guarded in macroexpand_1. This
saves us a few funcalls.
Merge branch 'master' into paultag/ana/if

Conflicts:
	hy/contrib/anaphoric.hy
Merge branch 'paultag/ana/if' of github.com:paultag/hy into paultag/ana/if
Merge branch 'olasd:feature/only-macroexpand-when-needed'