[feature-request] allow custom transients/commands

[TLINDEN]

(I'm sure it's not being called transients in gitu but I hope you understand it nevertheless)

It would be great to be able to add commands as new transient window or add subcommands to existing transients.

For example I have this snippet configured in my emacs config for magit. It adds an s key to the l (log) transient and executes dired. Essentially by using this I can just enter ls in the magit main buffer and get a directory listing - while staying in magit.

Another thing would be to not add new commands but modify existing ones, e.g. the look of the git log output.

[altsem]

Related: #61