Allow whitespace in cmd_spawn/cmd_output args

[NiLuJe]

Fix #16

(Mildly tested, feel free to try to break it ;)).

[pgaskin]

I'm testing it using these items:

# pid should not show
menu_item:main:spawn1:cmd_spawn:quiet:sleep 5
chain:dbg_msg:Success
# pid should show
menu_item:main:spawn2:cmd_spawn:sleep 5
# /tmp/nm should be asd, pid should show
menu_item:main:spawn3:cmd_spawn:quiet_ || echo asd > /tmp/nm
# /tmp/nm should be "asd:echo dfg", pid should show
menu_item:main:spawn4:cmd_spawn:echo asd:echo dfg > /tmp/nm
# /tmp/nm should be "asd:echo dfg", pid should not show
menu_item:main:spawn5:cmd_spawn :  quiet  :  echo asd:echo dfg > /tmp/nm