Add ability to change rhs color, change configuration options

[oldwomanjosiah]

• Added ability to color rhs

" 'left', 'right', 'both' for color, any other value for neither
" 'left' is default if left undefined
let g:lightline_gruvbox_color = 'both'

• Changed g:lightline_gruvbox_style to only react to 'hard' and 'plain,' though I consider 'plain' deprecated.

[shinchu]

Hi! Thank you for your contribution. I will consider merging based on backwards compatibility. Anyways, your forked repo would be very helpful for those in need of the function.