Command line support for <c-u>

Allows the clearing of the text from the caret position to the start
Changed value of CurrentSearchText

The CurrentSearchText property used to include both the search prefix (/
or ?) + the value being searched for.  This was inconsistent with the
name and other similar properties.  Changed it to actually return the
current search text
Call command supports local prefix

Added support for the local function prefix to the :call command.  This
doesn't actually allow for calls to occur.  It just allows VsVim to give
a better error message in a larger number of scenarios