Adding basic statusline support for async command.

Adding basic statusline information for running async commands. Access
by calling asynccommand#statusline(). Access current job dictionary with asynccommand#current_commands(), which could be extended to support alternate displays.

Initial commit towards https://github.com/pydave/AsyncCommand/issues/19.

Fixing bugs in grepprg and makeprg expansion. '$*' should be expanded to the :make and :grep argument list if given in makeprg/grepprg/.