Expose 'selection and yank' mode

Hi,

Thanks for keysnail, firefox the emacs way! Lovin' it!
Thanks for HoK, way faster than mouseless ever was!

Could you consider the yank-mode a worthy functionality to expose directly?

Here is the actions I did to do so:
- create the yank function
- expose it through a yank mode
- renamed the anonymous function from the extended mode to use the yank function

I expose the c binding which stands for copy (inspired from the [conkeror binding](http://conkeror.org/))

Cheers,

PS: I do not write/speak japanese so the doc is surely wrong though!