standardize plugins, update metadata

add about-plugin metadata
chmod -x plugins
cleanup filenames to standardize on x.plugin.bash format
only plugin files intended to be executable from the command line should
contain a shebang line, and should be a+x.

plugins/available/extract.plugin.bash

@@ -1,3 +1,5 @@
+cite about-plugin
+about-plugin one command to extract them all...
 extract () {
   if [ $# -ne 1 ]
   then