Support autorepeat?

[anko]

As pointed out in #6, currently holding a key only displays when the key was pressed, and when it was released, with no autorepeat. For example, holding down a for a while with xkbcat -up produces

+a
-a

Since key autorepeat is supported at an X11 level, it would be convenient to have an -autorepeat flag, so that xkbcat -up -autorepeat could instead show

+a
=a
=a
=a
-a

where =a stands for an autorepeated KeyPress event.

This change would be backward-compatible.

The XI_RawKeyPress events currently listened for don't contain the flag, but XI_KeyPress events have event->flags & XIKeyRepeat.