Feature/email highlighting

In response to a bug report that a user was getting emails like this:

    @Cody - fwiw these folks *a*re loc*a*l *a*nd do *verification including d*a*t*a* from *DIA*: http://www.verifidentity.com/

From: https://fyi.org.nz/request/1440-does-dia-provide-a-verification-service-for-passports-to-companies#comment-470

The reason seems to be that the title includes the word "a" and xapian is passing it as a word to be highlighted. Made highlighting respect word boundaries, and added a few basic (English) stop words.

Truth be told I'm not sure why there's highlighting in emails.