User contribution of a custom header

Some people (me) might prefer a (arguably(?)) simple, nifty looking ASCII art instead of a cow.

I hereby contribute by sharing my custom header.
```
let ascii_art = [
\ ' __ _ ',
\ ' /\ \ /| | ',
\ ' \ \ \ || | ',
\ ' \ \ \|| | __ __ ',
\ ' \ \ \| |/\_\ /\ `¯¯`v¯¯`\ ',
\ ' \ \ |\/\¯\\ \ \¯\ \¯\ \ ',
\ ' \ \__| \ \_\\ \_\ \_\ \_\ ',
\ ' \/_/ \/_/ \/_/\/_/\/_/ ',
\ '']
```
Then set `g:startify_custom_header` to `ascii_art` or some other special combination of other elements; my line is `let g:startify_custom_header = startify#pad(ascii_art + startify#fortune#boxed())`.

Feel free to use or include in the plugin in whatever form seems appropriate.

License: WTFPL or whatever appropriate.

Start with g:startify_custom_header and the startify#pad/startify#fortune#boxed combination shown in the issue. Check how custom headers are currently handled, then confirm the contributed ASCII art can be included or documented without breaking rendering.

Written by the indexing model from the issue text.