Allow for searching within subtitle.

[corazzi]

This adds an additional option, ->searchWithinSubtitle(), which allows the user to search within the resource's subtitle.

In the case that I need it for, my client needs to be able to search for people by their name OR their company name (which is typically in the email address, so also the email address, by extension).

I could change the display value to allow for this, but keeping the subtitle looks better.