when a command is calling host.quit() in a D-Bus callback, GLib mainloop is exiting and quit the whole process without (apparently?) any way to block it. Couldn't find a good way to avoid that, so subprocess are now used as a workaround, instead of launching commands in the same process. When we'll get rid of python-dbus, there may be an other way.
Execuring external commands is now possible through the shell/! command.
This command launch jp in REPL mode, allowing do normal jp commands with some facilities.
Command can be selected with "cmd" (e.g. "cmd blog").
An argument can be fixed with "use" (e.g. "use output fancy").
Command is launched with "do", or directly with its name if it doesn't conflict with a shell command.
Arguments completion is still TODO (only shell commands are completed so far).
