Add several key-bindings for ex command line.

All completion-like bindings run tab-completion. Simple commands are
bound to their Emacs equivalences.

C-h is not bound because it is too usefull for accessing Emacs help.

This addresses #166.

--HG--
extra : amend_source : 52452a7a7ac7828e19f690942ff7a544a11a2826
Fix the "r" flag for `evil-ex-substitute'.

If the "r" flag is present and the pattern is empty, then either the
previous search pattern or the previous substitute pattern should be
used, whichever is more recent.

Furthermore this flag is implicitly set for :substitute commands with
argument (i.e. with pattern/replacement part).

This fixes #252.

--HG--
extra : amend_source : 635eb35ae299b2453e8011cc1cc37237e12e6e30
Add binding for C-v in ex state.

It does the same as C-q, inserting a quoted character.

This addresses issue #166.