Support --root for more hooks

[TNonet]

See: #432

I seem to get a docopt issue when specifying an option without any arguments. See: #429 (comment).
Therefore, I added <_ignored>... arguments as a workaround. I bet there is a better solution, but I wanted to get this initial commit out quickly.

I am unsure what is wrong with my new roxygenize tests.

[TNonet]

@lorenzwalthert, I can't seem to assign reviewers.

[lorenzwalthert]

ok @TNonet I think I found the problem. Quite annoying combination of here::here() as the default value for root (after setwd()`, it's not updated) and some weird {git2r} behaviour. Anyways, let's see if checks pass. Do you want to update the docs in the meantime? I.e. NEWS.md and the vignette on available hooks.