[Engine] Extract CommandModel parsing into a method
[Engine] Allow spaces in commands

Allow spaces in commands when parsing the data at the
CommandModel constructor. Now something like
/join chatroom "password with spaces"
is technically possible (this is relevant in several
places for XMPP).
[Engine] Fallback to simple command parser if new one fails
Merge pull request #140 from knocte/parse_double_quote_commands