Created a factory to properly inject MediumEditor.

[austinthedeveloper]

In certain circumstances, jshint will throw an error in applications where a plugin has not been declared correctly in Angular. When it is injected as a factory and called when needed, jshint will stop throwing that error.

This solution is based from this Stack Overflow question about injecting jQuery and underscore:

http://stackoverflow.com/questions/17408977/inject-jquery-and-underscore-to-angular-js-component

[thijsw]

Thanks for this PR, has anybody time to look into this, @simison ?

[simison]

Very low on time right now, but LGTM (tho I didn't test this).

I also think Factory is the right way to inject external libraries. :thumbsup: