Granular inverse options in a single table

[ellisonleao]

Following #220 idea, we will introduce fine-grained bold options in the setup call

We will need to remove

invert_selection = false,
invert_signs = false,
invert_tabline = false,
invert_intend_guides = false,
inverse = true

and create a new reverse table

require('gruvbox').setup({
   reverse = {
     selection = false,
     signs = false,
     tabline = false,
     intend_guides = false,
     search = true,
     diffs = true,
     statusline = true,
     cursor = true,
     error = true
  }
})

[cryptomilk]

I would name it invert_colors instead of reverse.

Also it would be nice to have:

require('gruvbox').setup({
  font_style = {
    comments = { italic = true },
    keywords = { italic = true },
    functions = { bold = true },
    variables = {},
  },
})