Make stemming language configurable.

This change allows you to configure the language used for stemming,
which should result in better search results for users with
predominantly non-English mail.

This change is backwards-compatible - the default stemming language
remains English, unless a specific language is set in your config.yaml.
To do so, add the key :stem_language to you config.yaml with one of
these values:

  danish, dutch, english, finnish, french, german, hungarian, italian,
  norwegian, portuguese, romanian, russian, spanish, swedish, turkish

IMPORTANT: after changing the language, you must re-index all of your
mail!

See http://xapian.org/docs/stemming.html for more information on how
Xapian performs stemming.
Added new hook 'gpg-expand-keys'

sup does not support GPG groups, since GPGME ignores group definitions
in gpg.conf by design. Therefore, there is no way e.g. to send encrypted
mails to mailing lists using GPG groups.

The new hook receives the list of current recipients and allows the user
to swap recipients with other email addresses or key IDs. A simple hook
to add group support could look like this:

----------8<---------------
lookup_table = {}

`gpg --with-colons --list-config group 2> /dev/null`.each_line do |line|
  email_and_keys = line.split(':')[2..3]
  lookup_table[email_and_keys[0].gsub(/[<>]/, '')] = email_and_keys[1].strip.split(';')
end

recipients.map { |r| lookup_table.has_key?(r) ? lookup_table[r] : r }.flatten
---------->8---------------
Merge pull request #6 from ericweikl/stem-language

Make stemming language configurable
Merge pull request #7 from ericweikl/gpg-expand-keys

Add new hook 'gpg-expand-keys'