Support for image captions

[rachelmcr]

ADMIN EDIT: This issue was replaced by this one.

Aztec currently does not support captions; the caption shortcode is displayed in plain text:

For comparison:

In the Visual editor, the caption shortcode was supported by displaying the caption in small, italic text below the image:

In Calypso, the caption is displayed in a smaller font with a blue background:

Tested on iPad Mini, iOS 10.2.1, WPiOS 7.0.0.20170214

[diegoreymendez]

@rachelmcr - Just tagged this as v2, since shortcodes are on hold for now. At least until we can deliver the higher priority fixes and features.

That said I'm looking forward to revisiting the priority of shortcodes as soon as the base work is complete (and ideally for inclusion in v1).

[diegoreymendez]

As a side note, this issue is related to #115, where we should (at some point) come up with a list of higher priority shortcodes we want to include.