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

river/command.zig

@@ -103,6 +103,7 @@ pub const Error = error{
     InvalidRgba,
     InvalidValue,
     UnknownOption,
+    ConflictingOptions,
     OutOfMemory,
     Other,
 };
@@ -136,6 +137,7 @@ pub fn errToMsg(err: Error) [:0]const u8 {
         Error.NoCommand => "no command given",
         Error.UnknownCommand => "unknown command",
         Error.UnknownOption => "unknown option",
+        Error.ConflictingOptions => "options conflict",
         Error.NotEnoughArguments => "not enough arguments",
         Error.TooManyArguments => "too many arguments",
         Error.Overflow => "value out of bounds",