Add variable `evil-ex-point'

This variable records the position of point before an ex command is
called. The reason is that (point) is usually moved in order to
reflect the current ex region.
Fix position of point when evaluating an <addr> argument (re #271)

--HG--
extra : amend_source : 7f7ead0dc04f059b5393808965a7aba1579af6cc
Add :t as alias for :copy (re #271)