Add a variable allowing to set the transition time

[polosson]

At first, thank you and congrats for this awesome little project.

I was looking for a variable in 01_settings/_variables.defaults.scss allowing me to reduce the transition time of the changes when hovering a button or a link, for instance... But to no avail.

It would be great to have such a variable in the settings, because I found that 1s is a bit too much.

Thanks in advance! :)