Added flinch to durations.css (#548)

I'm using these to time transitions and regularly use something in the 300ms range.

Author: mneumegen

src/extra/durations.css

@@ -17,6 +17,7 @@
     --work-week: calc( 5 * var(--day));
 
     --blink:           .1s;
+    --flinch:          .3s
     --sneeze:          .5s;
     --brief-moment:    15s;
     --pause:           10s;