Embedded plantuml diagrams in docstrings

[tom-cook-pmev]

This PR adds an optional ability to embed plantuml diagrams in the description section of endpoint docstrings. Example:

@app.route('/foo')
def foo():
    """
    # Expected sequence
    @startuml
    client -> server: /foo
    server --> client: response {id}
    client -> server: /bar/{id}
    @enduml
    """
    return 'foobar'

This relies on having a PlantUML server available. By default, it uses the public server available at http://www.plantuml.com/plantuml/img/ but this can be configured by setting FLASK_SWAGGER_PLANTUML_SERVER in the flask app config.

The diagrams it generates go into a subdirectory of the app's static content directory. By default this subdirectory is called uml but it can be configured by setting FLASK_SWAGGER_PLANTUML_FOLDER in the flask app config.

The diagram content is replaced with a GFM image tag. If an error occurs while generating the image, the image content is replaced with an error message. If the plantuml package is not available, swagger generation proceeds as normal.

I've also added parameters to the swagger method for title, version and description. This is mostly so that the description content can be scanned for diagram content.

Currently uses some Python 3.6+ syntax. Let me know if this is an obstacle to merging & I can fix.