Add markup language named willow ( .willow .wl )

There's too many syntax rules in Markdown. I want something as simple as plain text,
but with `code` ,  **bold** and [link](http://github.com) support.
Sounds familar to gfm,, but gfm is not supported here Y_Y
Here's what `willow` looks like when putting in Github readme:
https://twitter.com/jiyinyiyong/status/246214596782858241/photo/1/large