Add g:gundo_playback_delay option

Allow for the option for setting the delay (in milliseconds) to be used
by the GundoPlayTo function in calling vim's sleep command during play
back.
Update inline help message to show real keymaps.

Instead of displaying hardcoded "j/k" for motion between undo states,
real keymaps are fetched from `g:gundo_map_move_older` and
`g:undo_map_move_never` variables.
Update changelog.
Merge.
Update changelog.
Fix version number in changelog.  yay semver.
Added tag v2.5.0 for changeset 39f23f084564