Update company-mode backend

* The Conventions section was inaccurate.

* Called interactively, a backend is supposed to initiate completion.

* Keys in `case' clauses don't need to be quoted.

* Using `company-completion-finished-hook' instead of `post-completion' command
  messed up completion with other back-ends.

* The `crop' command is new (added in company-mode 0.6.1), it makes sure that
  `company-complete-common' won't complete too far (it can insert the opening
  bracket and some function arguments otherwise). Users of the previous versions
  just won't receive this benefit.
company-mode backend: implement 'interactive command
We neeed thingatpt to be loaded