Add support for threshold-based script invocation

[dvejmz]

This PR provides a working MVP solution to close #25.

The following functionality has been added:

• Created application config directory in $XDG_CONFIG_HOME/s-tui or ~/.config/s-tui if $XDG_CONFIG_HOME is not set.
• Create hooks.d directory in app config location to add threshold scripts.
• Add base logic for threshold-based script invocation in the form of hooks
  • Create Hook base class. This should become the base class for all types of hooks.
  • Create ScriptHook wrapper class to run shell scripts located under hooks.d
    • Add functions to Source parent class so that hooks can be added to any source
• Add TemperatureSource script hook.
• Update READMEs with user instructions on how to add their own scripts

Enhancements for future iterations not included in this PR:

• Ability to change default config directory
• Ability to change hook invocation timeouts (right now it defaults to 30 seconds)
• Add hooks to more sources

[amanusk]

Thank you for your contribution. This looks great. I was thinking to add a config file. It will be good bases to also configure s-tui itself. (e.g what sensors/graphs to show). I will test this on a few systems before approving