Decouple toggle linting keymap from plugin config function

[peter-bread]

Move some of the linting configuration into util.

Then use vim.g.linting_enabled to store state (naybe).

This could also be treated as its own plugin, but in this repo.

With both of these changes (and #93) I should be able to decouple the toggle linting keymap from the config function.

[peter-bread]

Also jsut realised I use both enabled_at_start and enabled???