Bootstrap WebmentionIO with webmention command

[ashmaroli]

Since jekyll webmention command is going to be used separately from jekyll build or jekyll serve, we can initialize a new Jekyll::Site instance for the plugin's command.

Resolves #115

P.S. @aarongustafson if you can "describe" how a sample site would look when this command is to be used, we can setup Travis to test this class as well..

[aarongustafson]

There are two components to it that would be testable. Both assume a post that will trigger a mention (i.e., it contains a link to another site).

1. The webmention_io_outgoing.yml file will contain the link, referenced by the post in a format similar to this at the start:

https://www.aaron-gustafson.com/notebook/links/talking-to-the-web/:
  https://www.youtube.com/watch?v=H736Y3U3cmE: false

That indicates the mention has not been sent. Once it is sent, that will be changed to

https://www.aaron-gustafson.com/notebook/links/talking-to-the-web/:
  https://www.youtube.com/watch?v=H736Y3U3cmE: ''

OR (if the response is JSON) it will be changed to a string version of the JSON response.

2. The output of the command line should come back with a count of webmentions sent.

[aarongustafson]

Ran into a conflict with the original refactor of WebmentionIO::Webmention and the Webmention gem which takes care of sending webmentions & such. Had to rename the internal class to WebmentionIO::WebmentionItem.

[ashmaroli]

Ran into a conflict with the Webmention gem

That's a nice move. Always better when we can differentiate ourselves from our dependencies..