Only blur element if it's contained by one of the selectors

Previously, Pjax would blur (remove focus) from the active element
regardless of where it was on the page. This restricts that to
happen only if the active element is contained by one of the
elements represented by options.selectors, because only those are
affected by Pjax.

Fixes #4

lib/util/contains.js

@@ -0,0 +1,12 @@
+module.exports = function contains(doc, selectors, el) {
+  for (var i = 0; i < selectors.length; i++) {
+    var selectedEls = doc.querySelectorAll(selectors[i])
+    for (var j = 0; j < selectedEls.length; j++) {
+      if (selectedEls[j].contains(el)) {
+        return true
+      }
+    }
+  }
+
+  return false
+}