Add single file HTML and MD output of spec and store it in each ERC

[nuest]

use https://github.com/jgrassler/mkdocs-pandoc to create single file, then https://github.com/jgrassler/mkdocs-pandoc#usage-example and http://pandoc.org/demos.html to create

• single Markdown document output of spec is part of Travis build @nuest
• the version of the spec is available at http://o2r.info/erc-spec/erc-spec-v1.md @nuest
• new content for spec where in the ERC the actual spec is stored (integrate with spec_version!) > Package slip for archival extension #10
• Automatically download (see URL above) and (a) save the latest spec version into ERC during creation in extractor and (b) creates a file .erc/erc_raw.yml @7048730
• PDF is at http://o2r.info/erc-spec/erc-spec-v1.md
• unversioned PDF and single file markdown at http://o2r.info/erc-spec/erc-spec.pdf respectively http://o2r.info/erc-spec/erc-spec.md

[nuest]

@7048730 should we store the spec within the ERC as a PDF or as an HTML file?

[ghost]

how about PDF/A for long term preservation and additionally markdown for usability?

[nuest]

PDF/A with pandoc... that could be a challenge :-). I didn't find anything in that direction, so any pointers are helpful.

An what about the file names and paths?

[ghost]

cf. jgm/pandoc#3215

Since PDF/A seems problematic, I'd go for a text version that is as much plain text as possible. On the other hand, as we do use a lot of hyperlinks, we might be okay using markdown anyway, which is after all only very lightwightedly encoded and thus easy to preserve.

As for the filenames and paths, it might be reasonable from a preservationist's point of view to include the different spec outputs in directories that label them, e.g. erc/spec/markdown/spec.md and so on. At a point, one could add addtional information on the formats used here within that dir.

[nuest]

No PDF/A for us, paths to the plain text documents are listed in the archival metadata, see #10