added plantuml_rst plugin

Added a plugin that defines a 'uml' directive to be able to use PlantUML
diagrams with rst documents.
Merge pull request #212 from jkrshw/bugfix/rep-image-readme

[representative_image] Update README, including BeautifulSoup requirement and usage.
Merge pull request #214 from mikitex70/master

Add plantuml_rst plugin
Add GZIP_CACHE_OVERWRITE option to overwrite original files

This option is useful when hosting static content on a service such as
Amazon S3. Setting to True will overwrite the original file with
the gzip-compressed file. In order to correctly serve the file, the
Content Encoding must be set to gzip by the hosting service.
Merge branch 'feature/gzip-overwrite' into custom-plugins