Add ability to add custom node filter (or maybe combine ruletypes)

[pauljt]

We need rules that are only slighty complex to reduce false positives e.g:

ringtonePlayer.mozAudioChannelType = 'alarm' navigator.setMessageHandler('connect',callback) iframe.setAttribute('mozbrowser', true);

(literals, function calls and members all aren't very useful in this case)

A couple of ways to do this:

• supply some object-like thing which can be parsed and matched
• create a few more 'primitive' rule types, and then allow combination of rules (e.g. call function "foo" with arguments ["bar"])
• maybe looking acorn and acorn.walk for helpful matching functions (acorn.walk.findNodeAt/findNodeAround/findNodeAfter)

Or maybe we need a different algorithm than just test each node, one by one.