Wrap with star or underscore

[infinitebuffalo]

Simply takes the corral-single-/double-/backquotes- code and copies it to provide corralling with star or underscore.
Useful perhaps most obviously for Markdown users, I suppose.

Suggested keybindings (aka, what I use myself):
(global-set-key (kbd "M-*") 'corral-star-forward)
(global-set-key (kbd "C-M-*") 'corral-star-backward)
(global-set-key (kbd "M-\_") 'corral-underscore-forward)
(global-set-key (kbd "C-M-\_") 'corral-underscore-backward)