Allow a node matching function in bufferRangeForScopeAtPosition

src/selectors.js

@@ -22,6 +22,7 @@ const always = scope => true
 // true iff the scope matches the selector.
 function matcherForSelector (selector) {
   const parts = parse(selector)
+  if (typeof parts === 'function') return parts
   return selector
     ? scope => isSubset(parts, parse(scope))
     : always