Update shortcode functionality to make it easy to insert anchor links

[crbdev]

As discussed in our Slack channel, let's add the ability to use shortcodes in the following way:

[gravityexport id="5" class="some-class"]Text here[/gravityexport]

The text between the opening and closing shortcodes outputs as an anchor link to the report, as defined by the id value.

The only shortcode at this moment is this one:

If we had a proper shortcode we would be able to use it on GravityView: https://github.com/GravityKit/GravityExport/issues/47