Update API documentation

[S-S-X]

Update API_REFERENCE.md and README.md to include latest additions and changes.

• Updated metatool/api.lua documentation
• Documentation for metatool/formspec.lua formspec API
• Documentation for metatool/command.lua chat command API

If needed add new markdown files for command / formspec documentation, could maybe add docs directory, drop files there and cleanup README.md to only contain very basic overview of metatool and maybe links to documentation files.

[S-S-X]

Updated some small things, WIP branch for documentation: https://github.com/S-S-X/metatool/tree/documentation