v0.3

Adds the following features:

* Automatic numbering of figures including user-changeable text prefix
    and -appendable CSS class
* Support for images inside of links
* Support for newer Markdown versions

Furthermore, attributes set by other plugins may be preserved (for
example from `attr_list`), depending on those plugin's configuration.

Philipp Trommler (10):
      requirements.txt: Allow for newer Markdown versions
      Add the possibility for figure numbers
      Allow custom classes for figure numbers
      Make the figure number title user-changeable
      Provide IDs for figures
      Preserve attributes set by attr_list
      Support images nested inside of links
      Refactor
      Update copyright years
      Release v0.3