Feature: support path expansion with custom paths

[clarkadamp]

I was an early adopter of this plugin and recently updated it and it broke my use of custom paths which allowed glob style expansions.

Many of my projects have seperate modules under a top level project name, using something like

set -g @sessionx-custom-paths "$HOME/nested-projects/*/modules/*"

used to work well.

@sessionx-custom-paths-subdirectories kind of did what I wanted, but having complete control over subdirectories is even better.

• Used the same functionality as subdirectories, but only list direct matches
• Updated the README example to include double quotes to show that it is possible for environment variable expansions in tmux.conf