This is a limited implementation, so please backup your database before
running this search feature branch from now on as we may change things.

It's using a Unicode Snowball stemming tokenizer available from
https://github.com/littlesavage/sqlite3-unicodesn, also handily
available in src/sqlite3-unicodesn in Geary.  If you want to look at the
search tables on the command line, cd into the unicodesn source folder,
run make and make install, then load sqlite3 like:

   sqlite3 -cmd '.load unicodesn.sqlext' /path/to/geary.db