CSS Helper Positioning Class - .absolute-center (quasarframework#987)

* Update positioning.styl

* Update positioning.styl

Author: SeregPie

src/css/core/positioning.styl

@@ -20,6 +20,7 @@
 
 .absolute,
 .absolute-full,
+.absolute-center,
 .absolute-bottom,
 .absolute-left,
 .absolute-right,
@@ -66,6 +67,11 @@
   bottom 0
   left 0
 
+.fixed-center, .absolute-center
+  top 50%
+  left 50%
+  transform translate(-50%, -50%)
+
 .vertical-
   &top
     vertical-align top !important