CommandRunner.parse had some logic for handling commands of form
:<count>:cmd. However, this complicated the parsing logic for something
that appears to only be used in tests. One could use it in a
userscript, but this is unlikely as it is undocumented. Removing
support for this simplifies the logic of parse.
The commnd `run-with-count` is added to provide this functionality.
It works like `repeat` but passes the count along to the command
instead of running the command multiple times.
This resolves#1997: Qutebrowser crashes when pasting commands.
This bug was caused by excess stripping of ':' from the command string
by _parse_count.
