Add new username rule for Github

[seeeturtle]

This is added since Github cannot start or end with hypen

Closes https://github.com/manu-chroma/username-availability-checker/issues/33

[seeeturtle]

wow! @andrewda, I didn't even think that you're still watching this repo :)

[andrewda]

Heh, I've always been watching, just been super busy 👍

[manu-chroma]

Thanks guys! :)