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 #7 from ericweikl/gpg-expand-keys

Add new hook 'gpg-expand-keys'