Add a command-line option to override the default log file path

[acuteenvy]

Is your feature request related to a problem? Please describe.
gitui --logging always writes to ~/.cache/gitui/gitui.log. There is no way to change the location of the log file.

Describe the solution you'd like
Some kind of command-line option to override the default path.
gitui --logging --logfile <path_here> for example.

Describe alternatives you've considered
Symlinking ~/.cache/gitui/gitui.log works, but I'd like to put the log file in the current working directory, and that's not possible with a symlink.

Additional context
I can send a PR if this is something you'd like to support.