Extract enter and exit focus mode functions.
Replace hard coded string with an auto variable.
Add mksession options and tab handling.
Add temp file for sessions. Add debugging helpers.

A temp file is now used for session saving.
Several minor changes allowing the plugin to be reloaded at
will to add debugging and development.
Minor variable scope refactoring.
Add saving and restoring cursor position.
Fix a bug in sourcing the saved session file.

Minor variable scope cleanup.
Quit Vim when closing focused buffer.

Fix a bug causing key binding not to be executed.
Make entering with single buffer more streamlined.