mavproxy_cmdlng added and cmd guided with lat/lon param upgraded

Hi,

We have added a **`mavroxy_cmdlong`** module to manage MAVLink **COMMAND\_LONG** request like **MAV\_CMD\_DO\_CHANGE\_SPEED**.

Moreover, we have upgraded, the **`mavproxy_mode`** module and the ***cmd\_guided*** function can now be run by taking the GPS position in command line.

Bye,

MousS'