Introduce the `Figure` identifier to add captions to images

[tobiasah]

Allow a custom markdown identifier for all types.

This PR adds a config parameter markdown_identifier which allows changing the identifier this plugin looks for when parsing a markdown file.

Since the images did not support this kind of syntax it was introduces as well.

Figure: Captions {<figure properties>}

![](img.jpg)

This change was necessary to allow adding attributes to the figure element directly. Previously this was only possible for the img elements directly.

Fixes #1