Allow big group matching for "scale" special color

When matching a group with "scale", filter out everything that does not seem to
be necessary to interpret the string as a number this permits to transform
"[95%]" to "95" before number conversion, and thus allows to color a group larger
than the matched number

Update the cmake theme accordingly.

delete duplicate entry for `--all` in the README

bugfix nojhan#40: don't try to color non-matching groups

Remove on_stderr from the README