Feature: filter on expansions

[danielb2]

Because of issue #1 I took a look at the craigslist peak (https://chrome.google.com/webstore/detail/knpehhedikdgkbmhgagpcpcbclaidlmf) plugin.

I like the filter function in that extension. Any chance you could implement that?

And make both a positive/negative filter too, instead of just a negative.
i.e. Being able to filter to show ONLY items matching keywords, instead of filter to not show items matching keywords.

make sense?