Add style override mechanism. (#70)

Reworked from #67; thanks @webdesus!

README.md

@@ -114,6 +114,30 @@ Place `onedark.vim/autoload/airline/themes/onedark.vim` in your `~/.vim/autoload
 let g:airline_theme='onedark'
 ```
 
+## FAQ
+
+### How can I customize onedark.vim's look without forking the repository?
+
+onedark.vim exposes a function called `onedark#set_highlight` that you can call from within your `~/.vimrc` in order to customize the look of onedark.vim by overriding its defaults.
+
+The function's first argument should be the name of a highlight group, and its second argument should be style data.
+
+For example, to remove the background color only when running in terminals (outside GUI mode and for use in transparent terminals,) place the following lines **before** the `colorscheme onedark` line in your `~/.vimrc`:
+
+```vim
+" onedark.vim override: Don't set a background color when running in a terminal;
+" just use the terminal's background color
+" `gui` is the hex color code used in GUI mode/nvim true-color mode
+" `cterm` is the color code used in 256-color mode
+" `cterm16` is the color code used in 16-color mode
+if (has("autocmd") && !has("gui"))
+  let s:white = { "gui": "#ABB2BF", "cterm": "145", "cterm16" : "7" }
+  autocmd ColorScheme * call onedark#set_highlight("Normal", { "fg": s:white }) " No `bg` setting
+end
+```
+
+More examples of highlight group names and style data can be found in onedark.vim's source code (`colors/onedark.vim` inside this repository).
+
 ---
 
 Preview images were taken using:

colors/onedark.vim

@@ -98,6 +98,14 @@ function! s:h(group, style)
     \ "cterm="   (has_key(a:style, "cterm") ? a:style.cterm    : "NONE")
 endfunction
 
+"public {{
+
+function! onedark#set_highlight(group, style)
+  call s:h(a:group, a:style)
+endfunction
+
+"public end }}
+
 " +-----------------+
 " | Color Variables |
 " +-----------------+