grimshot: Avoid screenshots overwriting each other

Due to the date format used, if several screenshots are taken in
succession, each one overwrote the other.

This change set makes each one have a different name to avoid this.

Also avoid using spaces, since many scripts and tools are unhappy with
file names with spaces.

contrib/grimshot

@@ -28,7 +28,7 @@ getTargetDirectory() {
 
 ACTION=${1:-usage}
 SUBJECT=${2:-screen}
-FILE=${3:-$(getTargetDirectory)/$(date +'Grimshot %Y-%m-%d %H-%M-%S.png')}
+FILE=${3:-$(getTargetDirectory)/$(date -Ins).png}
 
 if [ "$ACTION" = "usage" ] ; then
   echo "Usage:"