Save single session

[maybeetree]

Dear maintainer,

Thank you for your work on tmux-resurrect and for taking time to consider my pull request. In short, it adds the following features:

1. scripts/save.sh now allows saving only a single session, using the -t parameter. For example: ./save.sh -t my_session
2. There is now a new keybind, C-t, which saves only the currently focused session. This keybind is documented in docs/custom_key_bindings.md and README.md
3. When using the above features, the state line is not written to the save file. This is to avoid errors when restoring, in case the session that was in focus when saving is different to the session that was saved.

Unfortunately, there is one breaking change: due to the nature of getopts, suppressing the output of save.sh is now done with the -q flag (i.e. ./save-sh -q), instead of the quiet argument.

This pull request should resolve issue #427 .