Add argument to prepend dir to FFOO_PATH

to easily test out documentation for currently developed library.

bin/ffdoc

@@ -6,9 +6,16 @@ ffoo import pretty
 
 
 usage() {
-    usage_is "module[.function]"
+    usage_is "[-I directory]... [--] module[.function]"
 }
 
+while true; do case "$1" in
+    -I|--include) FFOO_PATH="$2:$FFOO_PATH"; shift 2 ;;
+    --)                                 shift; break ;;
+    -*)                                        usage ;;
+    *)                                         break ;;
+esac done
+
 target=$1
 test -n "$target" || usage
 ffoo doc "$target"