Implement Custom Keybindings

[CogentRedTester]

This system replaces the jumplist and allows users to set
custom keybinds for the different pages. These keybinds
send standard input commands, but users can use special
string codes which the script will substitute for information
about the currently selected item.

This will be most useful for the option and property pages,
but the current jumplist commands for keys and commands
will still be supported by the new system.

• File IO and JSON parsing
• Keybind declaration on page open
• Keybind page codes
• Command page codes
• Option page codes
• Property page codes
• Support mp.command style commands for the keybinds page
• Support argument parsing for cmd page
• Flag to temporarily hide the page
• Flag to close the page
• Custom parsing for option choice output (supports cycle-values)
• Filtering options by option type
• Filtering keybinds by whether they are overwritten
• Provide example/default config file
• Add documentation to the README
• Add new custom_keybinds option to example config
• Remove all mention of the jumplist from README