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-which

@@ -1,4 +1,8 @@
 #!/usr/bin/env bash
+# Usage: rbenv which COMMAND
+# Summary: Display full path to a binary
+# Help: Displays the full path to the binary that rbenv will execute when you
+# run the given command.
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x
 
@@ -44,7 +48,7 @@ RBENV_VERSION="$(rbenv-version-name)"
 RBENV_COMMAND="$1"
 
 if [ -z "$RBENV_COMMAND" ]; then
-  echo "usage: rbenv which COMMAND" >&2
+  rbenv-help which | head -1 >&2
   exit 1
 fi