Please and an Option for --first-parent

I would like to only send mails for merge commits but not the included commits.

diff --git i/lib/git_commit_notifier/git.rb w/lib/git_commit_notifier/git.rb
index eefbb9e..8ecef2b 100644
--- i/lib/git_commit_notifier/git.rb
+++ w/lib/git_commit_notifier/git.rb
@@ -186,7 +186,8 @@ class GitCommitNotifier::Git
       # We should now have ^B1... ^oldrev newrev

       # Get all the commits that match that specification
-      lines = lines_from_shell("git rev-list --reverse #{a.join(' ')}")
+
+      lines = lines_from_shell("git rev-list --first-parent --reverse #{a.join(' ')}")
       lines.to_a.map { |l| l.chomp }
     end

This is how I achived it. I would send a pull request but I wasn't sure how to access the config without adding a new param to new_commits, which probably isn't the right way of doing this.