Add new_draft_template option to config

[rmm5t]

Just like the new_article_template option in middleman-blog, it would be great to be able to specify our own draft template instead of using the default draft.tt that's hardcoded in the gem.

(Potential) Example Usage:

activate :drafts do |drafts|
  blog.new_article_template = "source/drafts/draft.tt"
end