Convert tmux variables to shell variables

[joshmedeski]

I originally started customizing this plugin using tmux variables which can't be read if tmux isn't running. So I switched to using shell variables (ex: T_SESSION_NAME_INCLUDE_PARENT) for some customizations in order to fix this issue.

This has left the user with two different places to customize the plugin (tmux variables or shell variables), which I think creates additional complexity for managing this plugin.

After some thought, I believe it's best to drop all tmux variables in favor of shell variables, here is my reasoning so that all customizations are in one place and work whether or not tmux is currently running.

I've also been able to simplify the README and show all configurations together, hopefully making it easier for people new to the shell and tmux to be able to approach customizing this shell to their needs more easily.

The added bonus is the bash code can use a fallback value inline during assignment, reducing the code significantly.

🚧 BREAKING CHANGE 🚧

The following tmux variables are dropped in favor of new shell variables.

• @t-fzf-prompt becomes T_FZF_PROMPT
• @t-bind becomes T_BIND
• @t-fzf-default-results becomes T_DEFAULT_RESULTS

[joshmedeski]

I'm open to feedback on these questions.

• Should I deprecate the current options and give people time to make the switch?
• Should we keep any tmux-only configurations as tmux variables?
• Are there any configurations that need to be changed?
• Is the updated README clear and easy to understand?

[joshmedeski]

I'm going to rework this so there are less variables you have to handle and you can just pass any/all fzf flags in one variable.