Add custom kind information to show where completions came from

[tu500]

Also return a completion kind of History to show up as type information in the nvim-cmp popup, to show where these completions came from. This improves of the default where these just show up with the kind Variable.

I'm not sure I used the right place to initialize the new highlight group.

[dmitmel]

Hey. Sorry that I did not respond on time. I've been away from nvim-cmp for a long time, is your implementation a commonly accepted practice? Also, can it be done by the user in their config?