Publicize: Add custom templates

We could add an additional option under Settings > Sharing, allowing you to customize all Publicize excerpts at once, with the use of template tags, like we do today for custom services in Sharedaddy.

Here are a few examples:

- Custom Text, like "New Post:" before the post title
- Tags added as hashtags for Twitter and Facebook
- We could do this with an interface similar to the old YOURLS plugin: http://i.wpne.ws/LFEe/content

Suggested by users in:
https://twitter.com/walkenfan/status/291950686277627904
http://wordpress.org/support/topic/featured-image-of-post-not-showing-up-when-publicized-to-facebook?replies=23#post-3467643
http://wordpress.org/support/topic/publicize-custom-post-text?replies=3#post-3556200
http://wordpress.org/support/topic/feature-request-33?replies=1
http://wordpress.org/support/topic/any-way-to-use-tags-when-posting-an-entry-to-facebook-twitter-etc?replies=1
http://wordpress.org/support/topic/share-on-twitter-feature-request?replies=2
http://wordpress.org/support/topic/publicize-add-automatic-title-hashtag-for-twitter?replies=3#post-4494444
http://wordpress.org/support/topic/jetpack-publicize?replies=1
http://wordpress.org/support/topic/publicize-setting-text-auto?replies=1

Ported from https://plugins.trac.wordpress.org/ticket/1785