Add variables for sidenav transition values

[robframpton]

When there is a fixed element on the page and I want it to transition with a sidenav, it's hard to figure out the transition duration and timing function used by the sidenav. So it would be useful to set those values with variables that can be used elsewhere.

So variables specifically for 0.5s and ease in _side_navigation.scss.

.sidenav-transition,
.sidenav-transition .sidenav-content,
.sidenav-transition .sidenav-menu-slider,
.sidenav-transition .sidenav-menu-slider .sidenav-menu {
    @include transition(all 0.5s ease);
}

[pat270]

Fixed by #176