Implement in-app help

[darylwright]

The in-app help will display in the 'Details' pane and appear when the <h> shortcut is pressed or the Help menu item is selected.

This documentation should also be in the README.md.

[darylwright]

I found out about ncdu far too late into this project 😅. However, ci has a number of advantages over it:

• can navigate above the directory it started in
• separates the list of directories from the list of files
• can do quick filtering
• is designed from the ground up to be cross-platform
• loads instantly and runs fast
• does one thing and does it well, which is navigate directories

There is one major disadvantage to ci and it's the dependency on external scripts to change the working directory in the parent process. This is due to ci not being able to access the parent process to call cd. I'll be looking into this via #14, but the current method of running ci seems unavoidable.

I should look to ncdu going forward to see what I may be missing in ci. The above pros and cons should be added to the README.md.