argv: introduce argv_format_arg()
display: expose status_win and opt_tty

This patch exposes the two variables (WINDOW *) status_win and
(FILE *) opt_tty, required by the incoming readline support.
Introduce the GNU Readline support

This patch introduces the readline(3) support to Tig, if it is available
on the system. That is to say, prompt history, cursor navigation and
completion.

The currently supported completions are variables (both "%(name)" syntax
and interpolation), actions (e.g. ":edit"), and options to the "set" and
"toggle" actions.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>