fts-lucene: Added initial support for language detection and stemming.

fts-lucene: Added initial support for language detection and stemming.
This requires textcat and snowball (libstemmer) libraries. This can be
enabled by setting:
fts_lucene = textcat_dir=/usr/share/libtextcat \
  textcat_conf=/etc/dovecot/textcat.conf

I'm not yet sure how good of an idea it is to actually use this, so it needs
some testing.. For example how bad is it if language is detected
incorrectly?