define bufferline colors for gruvbox

[cartwatson]

Gruvbox theme doesn't take advantage of specific bufferline colors and just uses defaults

Colors were chosen for the following reasons:

• bufferline
  • fg - default text color
  • bg - slightly darker than statusline, enough contrast to see tabs
• bufferline.active
  • fg - contrast, hard and soft contrast will change this
  • bg - arbitrary, just looked right of the available colors in the palette
• bufferline.background
  • bg - match statusline