Studio164a/dynamic submit fields

This set of commits opens up a whole lot of extra flexibility for the campaign creation page, which is something I need for a new plugin I'm working on. Without going into it too much, I wanted to be able to have two variations of the campaign creation page, with some different fields for each one. 

To summarise what I've changed: 

* Added shortcode identifier when calling `shortcode_atts`, which make the attributes filterable
* Passed `$args` array to `register_fields()` method, and then passing that along to the filter
* Removed the call to `register_fields()` in the `ATCF_Submit_Campaign` constructor, which doesn't seem necessary at this point

Cheers, 
Eric