command: support repeating keyboard mappings

Repeating mappings are created using the -repeat option to the map
command:

    % riverctl map normal $mod+Mod1 K -repeat move up 10

- repeating is only supported for key press (not -release) mappings
- unlike -release, -repeat does not create distinct mappings: mapping a
  key with -repeat will replace an existing bare mapping and vice-versa

Resolves #306

doc/riverctl.1.scd

@@ -185,11 +185,13 @@ A complete list may be found in _/usr/include/linux/input-event-codes.h_
 *enter-mode* _name_
 	Switch to given mode if it exists.
 
-*map* [_-release_] _mode_ _modifiers_ _key_ _command_
+*map* [_-release_|_-repeat_] _mode_ _modifiers_ _key_ _command_
 	Run _command_ when _key_ is pressed while _modifiers_ are held down
 	and in the specified _mode_.
 
 	- _-release_: if passed activate on key release instead of key press
+	- _-repeat_: if passed activate repeatedly until key release; may not
+	  be used with -release
 	- _mode_: name of the mode for which to create the mapping
 	- _modifiers_: one or more of the modifiers listed above, separated
 	  by a plus sign (+).