Add ability to filter to a specific context value outside the top 5

[itsderek23]

As mentioned in https://github.com/scoutapp/roadmap/issues/33#issuecomment-384824677 by @nathansamson, you can't discover values that lie outside the top 5 values of a specific piece of context.

Example need: a specific user reports an issue, but they don't access the site much so they don't appear in the list of top 5 users.

The current work-around would be to filter adjacent filters (like a specific endpoint), which would then filter down other context.

[itsderek23]

It looks like a text filter was added in v3 of dc-js:

http://dc-js.github.io/dc.js/examples/text-filter-widget.html