Fix performance regression introduced in 6.8.2

This commit e6c11a0 (#19) made a big performance regression.
The reason was that parseConditional was always cloning the current state
even if no question mark (potential conditional or flow-optional
token) was at the current position.
Simply checking if questionmark matches the current token solves the problem.

Fixes #62

test/fixtures/flow/optional-type/2/options.json

@@ -1,3 +1,3 @@
 {
-  "throws": "Unexpected token (1:12)"
+  "throws": "Unexpected token (1:13)"
 }