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/hg.plugin.bash

@@ -0,0 +1,25 @@
+cite about-plugin
+about-plugin hg helper functions
+
+hg_dirty() {
+    about displays dirty status of hg repository
+    group hg
+
+    hg status --no-color 2> /dev/null \
+    | awk '$1 == "?" { print "?" } $1 != "?" { print "!" }' \
+    | sort | uniq | head -c1
+}
+
+hg_in_repo() {
+    about 'determine if pwd is an hg repo'
+    group hg
+
+    [[ `hg branch 2> /dev/null` ]] && echo 'on '
+}
+
+hg_branch() {
+    about display current hg branch
+    group hg
+
+    hg branch 2> /dev/null
+}