add repeat command to make the same action again

The UI object now saves the last executed command (except for
RepeatCommands themselves and Promt- and FlushCommands, since they are
not user invoked, intended actions). The RepeatCommand then applies the
stored command again.
This is comparable to the . in vim, so I would recommend you add this
line to your config ;)

. = repeat
add '. = repeat' to default key bindings
remember and re-interpret command*line* for repeat

... commands. At some point we might want to add a command
history for an undo feature. For this I'd keep a history of
the applied Command objects and in reverse order call its .undo() method.
For this to work we need one Command object for each application.
If however we re-use a Command object using 'repeat' we'd most certainly lose
local information on how to undo the first application..
Merge branch '0.3.4-feature-repeat-534' into testing
add hooks [pre|post]_buffer_[open|close|focus]

cf issue # 563
document new hooks
fix: forgot buf parameter for post_buffer_open hook
Merge branch '0.3.4-feature-hooks-563' into testing