Fix bug when selecting indented proof blocks

[mateoxh]

The old code didn't handle the case where the proof block is indented. Maybe we should add tests for this?

[whonore]

Yes, some tests in tests/vim/ would be great. Something along the lines of:

# tests/vim/proof-block.vader
Given coq (simple-proof-i):
  Goal True.
  Proof.
    auto.
  Qed.

Do (simple-proof-i):
  diP

Expect coq (simple-proof-i):
  Goal True.
  Proof.
  Qed.

[mateoxh]

Writing the tests helped find a couple of bugs but also want to change the behavior in some edge cases, for now I think they're enough.