switch: add option to only include sessions from search paths

[junglerobba]

Not sure if anyone other than me would find this useful, but I sometimes I open unrelated tmux sessions for stuff in the background that pollute the switcher. To get to those there's always the option to use tmux's built-in switcher. Alternatively, tms could be made to support command line flags for its options that take precedence over the config file, so that it's possible to use separate hotkeys for both of versions of the switcher. Let me know if that's preferable.

[jrmoulton]

Can you add the config option to the config subcommand and update the README with the updated help output

Other than that this looks good to me

[jrmoulton]

Looks great!