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.
`checkmake` by default checks config file "in the same folder it's
executed in" unless `--config` option is set.
This commit allows setting the `--config` option with an option
variable (with documentation updated).