Text.Pandoc.Pretty:  added blanklines.

This ensures a certain number of blanklines (and no more) in output.
Markdown writer: More improvements to 'plain' output, updated tests.

Math now appears in unicode if possible, without the distracting
italics around identifiers.

Blank lines around headers are more consistent.

Footnotes appear in regular [n] style.
Markdown writer: Separate adjacent lists of the same kind with comment.

Closes #1458.
Parsing: Added isbn and pmid schemes
Merge pull request #1459 from mpickering/formats

Parsing: Added isbn and pmid schemes