tools (common/template): add filter to get media types: Add 2 filters to get main type and subtype of media type. Jinja2 and Nunjucks don't handle slices in the same way (Python way for Jinja2, JS way for Nunjucks), making it difficult to retrieve main type of a media from media type. Thoses filters work in both cases.

You'll find install instructions in documentation (see doc/installation.rst).

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