1. Add ale.setup and ale.setup.buffer for pure Lua configuration.
2. Update many global settings to use Booleans instead of numbers to
make types easiert to work with in Lua.
3. Radically reformat documentation and fix errors to make
documentation more usable for Neovim users.
* ale.txt: fix indentation
* ale.txt: fix Type and Default markup
* ale.txt: use `not set` instead of `undefined`
This matches the way the variables are referenced in prose about
existence state.
