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-rehash

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
+# Usage: rbenv rehash
+# Summary: Rehash rbenv shims (run this after installing binaries)
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x