Added a parser for MESA defaults files

A future version of MESA will include defaults files which will be
formatted according to a set of (in-progress) formatting guidelines.

This adds a script which can parse these files into markdown, which can
then be included into the main MESA site.  It requires the parslet gem.
Chomp paragraph symbols off of anchors
Refactor parsing scripts

The parser remains in defaultsParser.rb.  This moves the script that
transforms the parse tree into Markdown into markdownTransformer.rb.

A script that uses these to parse the MESA defaults files is included in
make_docs.rb.
Update pages that display defaults files

This removes the old defaults pages and the hacks that were needed to
get them working.  It adds individual pages for each of the new defaults
files.
Fix parser definition of "option"

In pgstar.defaults, an option can look like foo(:,:)
Drop trailing comments

This allows pgstar.defaults to be rendered as-is
Allow ToC to not be at the top

This adds the ability to put the ToC somewhere other than at the top of
the page.  If a div with id "toc-generator" exists, than the ToC is
placed there.
Don't escape underscores and asterisks

Instead of escaping underscores and asterisks, we now require that
entities contains these characters be `backquoted` in the defaults
files.  This has the additional advantage of identifying formulas and
variable names.
Newspost about new MESA SDK
Clarify what files to commit

The documentation markdown files are now generated from the *.defaults
files server-side at publication time.  There is no need to track them.
Update to release 6794

This has a placeholder URL for the mailing list announcement.
Copy over defaults files from new release
Update the defaults pages

This add some more explanation about the defaults pages and asks users
to send changes to Josiah.
Hide format page from sidebar

This uses tags (a bit of a hack) prevent the page that defines the
defaults file format from being listed in the sidebar.
Merge branch 'master' into maint

For release 6794