Generalized Cheat Sheet for user sheets to be added.

Cheat sheets can now be added to ./Packages/Users/cheat_sheets/filename.cheatsheet.

ST commands are now issued as `"command": "cheat_sheet", "args": {"cheatsheet": "filename"}` and there is a similar cheat_sheet_tester that will print whether or not filename could be found to the ST console.

I changed the highlighting and fixed a bug highlighting sections with spaces, and updated all the other files where necessary.