`rg` will now be used over `ag` if it is available. This is because `rg`
is a faster alternative reduces lag when searching larger codebases.
`ag` is also now conditionally loaded if the command is available.
We now have a concept of a 'backend' this is the same as the current
sorters and finders but with added info like the keymap so they can all
be registered as one. This will allow us to split our backends into
modues so we can better maintain then.
