Add configuration option for disabling mouse warping

This patch adds a new configuration option "mouse_warping [output|none]".

When mouse warping is disabled, mouse cursor does not jump to middle of current
screen when changing workspaces between multiple outputs. This introduces a
"special" cursor state, where focus is in one window and cursor on another.
Useful for eg. scrolling a web page with mouse wheel while typing into another
window on keyboard.
Refactor parse_command

parse_command returns a struct that contains useful information about
the result of a command as a whole (instead of the intermediate
representation used during parsing).

parse_command now requires the caller to allocate the yajl_gen used for
generating a json reply. This is passed as the second parameter to
parse_command. If NULL is passed, no json reply will be generated.