Create cache directory if it does not exist

[mschoettle]

When specifying a cache directory that does not exist mkdocs fails with the following error:

ERROR - Error reading page 'architecture.md': [Errno 2] No such file or directory: '.plantuml/1f370dd8.png'

mkdocs.yml excerpt:

  - plantuml_markdown:
      server: http://www.plantuml.com/plantuml
      # use caching to speed up live-reloading
      cachedir: .plantuml/

I would like to propose to create the directory if it doesn't exist.

[mikitex70]

Good idea. Implemented and released.

[mschoettle]

Thanks!