foundation for a help system where each command holds its own docs

Docs are comprised from "Usage", "Summary" and "Help" sections, where
"Help" can span multiple commented lines. If it is missing, "Summary" is
shown in its place.

References #204, references #206

libexec/rbenv-commands

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
+# Usage: rbenv commands [ --sh | --no-sh ]
+# Summary: List all rbenv commands
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x