Hotend offset handling for multi-extruder machines

Merged extruder offset handling from dob71/Marlin.

The extruder offset can be specified in the configuration file or adjusted on the fly using the "M218 T# X# Y#" command (T - extruder number, X,Y - offsets).

The "T#" (extruder change) command can now take option "F#" that specifies the feedrate
at which the printing head should be re-positioned. If not specified the re-positioning move is not preformed immediately, but the coordinates are adjusted for the printer to properly position the head when the next movement happens.
The EEPROM support for storing/retrieving these settings is not yet merged in.