Support per command cheat sheets

[ripienaar]

Applications should support opt-in cheat support:

app := New("x", "").WithCheats()
app.Cheat(`# cheat output here`)

cmd := app.Command("sub", "")
cmd.Cheat("# cheat output here")

This should add a x cheat command that works a bit like help but renders the cheat sheets, it should do a few things to behave cheat like such as x cheat sub/other should work to find the cheat for 2 level deep commands etc.

It should support saving them to a directory for use by the normal cheat command x cheat --save <directory> and if some cheat is asked that doesnt exist should list all the sections of cheats in x/y format

The command it creates should be accessible same as HelpCommand etc so users can modify it

• in-command access to cheats (#4) initial support for cheats #5
• integration with cheat cli