Add MQTT URL scheme support

Add option -L to specify user, password, hostname, port and topic at once.
The URL must provided with the -L or --url in the form:
mqtt(s)://[username[:password]@]host[:port]/topic

Change-Id: Ia626a28981a38807a254ae32c6ffb29b122b8a28
Signed-off-by: Matteo Croce <matteo@openwrt.org>