Add variables for transitions

Created by: tomlutzenberger

In order to standardize transitions it would be useful to add variables for transitions. When looking at transition usage, I see that in most cases ease-in-out is used as timing-function and 0.2s / 0.6s as duration. These values could be transfered into variables.

Additionally, the mixin could be extended to provide those variables as default values if no parameters are given. So using @include transition(); could end up in transition: all 0.2s ease-in-out; after compiling.

I would be happy to make those changes and open a PR 😃