Allow panflute filters to be autodownloaded from github.

[sergiocorreia]

Note: I think this is outside the scope of panflute, and there should be a separate repo that deals with this (we can add it to panflute's dependencies)

Implementation:

• The package should fetch a filters.yaml file from the repo of an organization repo. This file should have a list of items, each with two fields (name and url).
• A filter consists of a python file (for now let's limit ourselves to one file) and optionally a yaml file (e.g. headers.py and headers.csv)
• The YAML file contains metadata about the filter. This includes: name, description, author, date, version (!), etc.
• If at some point way in the future we want to extend this, we can do it easily through the yaml (eg: allow for more than one python file, more than just python files, versioning, example input and output markdown files).
• The url should point to either the .py file or the .yaml file
• Both files shoud be copied to datadir/filters (or to whatever path the user asks)

Finally, this utility should allow for a few suboptions:

• ??? install somefilter
• ??? update somefilter
• ??? install filter1 filter2 filter3
• ??? uninstall filter1

Also see:

• Bower: https://bower.herokuapp.com/packages
• Conda forge: https://github.com/conda-forge/feedstocks
• ST Package control: https://github.com/wbond/package_control_channel/

[sergiocorreia]

Also see previous discussions:

• Misc. ideas #12 (comment)
• https://groups.google.com/forum/#!topic/pandoc-discuss/wbebx65e1Nk

[sergiocorreia]

This has now mostly been implemented here:

pandoc-extras/pandocpm#2

So I'm closing the issue.