Feature request: expose highlight groups for floating window borders

[wurli]

Hi,

When Neogit is shown as a floating window, the background by default falls back to Normal, whereas the floating border falls back to FloatBorder. FloatBorder may have a different background colour to Normal, which can result in a slightly messy overall effect.

I would suggest:

1. Adding a NeogitFloatBorder highlight group which by default has the same background colour as NeogitNormal
2. Possibly also adding NeogitFloat, which would take priority over NeogitNormal in the event that a floating window is used.

I might be able to find some time for a PR if you like the sound of this :)

Many thanks for the great work on this plugin!

[CKolkey]

Sure, I'm open to a PR :)