Implement git flow feature finish without a branch name to finish the
current branch, if any.

This fixes #127.
diff --git a/git-flow-feature b/git-flow-feature
index ebbb035..2133330 100644
--- a/git-flow-feature
+++ b/git-flow-feature
@@ -232,7 +232,7 @@
 	DEFINE_boolean rebase false "rebase instead of merge" r
 	DEFINE_boolean keep false "keep branch after performing finish" k
 	parse_args "$@"
-	expand_nameprefix_arg
+	expand_nameprefix_arg_or_current
 
 	# sanity checks
 	require_branch "$BRANCH"