Avoid surprises with some commands accepting quoted arguments and some other
not.

fix #2555

use . instead of source
and add a var for the pyvenv command

it’s confusing.

if someone will ever read it

simplifies the script and avoid cluttering the poezio/ directory.

also fix an unrelated test with slixmpp

Conflicts:
	src/bookmark.py
	src/config.py
	src/connection.py
	src/core/commands.py
	src/core/core.py
	src/core/handlers.py
	src/windows/info_bar.py
	src/windows/muc.py
	src/windows/roster_win.py
	src/windows/text_win.py
	src/xhtml.py

Might fix some not-yet-seen bugs

(introduced by the automated default arguments of config.get)

- also fix that travis build

- we need to have more

Should work for any case (plugin, different section, etc)