LaTeX/Beamer templates: remove conditional around date.

Closes #1321.
Minor code reformat.
Docx writer:  Header and footer are now carried over from reference.docx.
Removed header1.xml, footer1.xml from reference.docx.
Docx writer:  Improved handling of headers/footers.
Updated README on reference.docx.
Updated tests for c8bc70a6bb12e0fbfadadb9480efafb3be4f11a5.
Templates:  Fail informatively on template syntax errors.

With the move from parsec to attoparsec, we lost good error
reporting.  In fact, since we weren't testing for end of input,
malformed templates would fail silently.  Here we revert back to
Parsec for better error messages.
Require latest texmath.  Closes #1319.

This allows `\left` to be used with `]`, `)`, etc.
Docx writer:  Make images work in reference.docx headers/footers.

* All media from reference.docx are copied into result.
* Added defaults for common image types to [Content Types].
* Avoided redundant XML parse + write for entries taken over from
  reference.docx, for better performance.
Docx writer:  Improved entryFromArchive to avoid parse.

No need to parse the XML if we're just going to render it
right away!
Docx writer: Create overrides per-image for media/ in ref docx.

This should be somewhat more robust and cover more types
of images.
Shared:  Added ordNub.

API change (adds export).
Templates: use ordNum instead of ord.

Closes #1022.
Version bump to 1.12.5.
Docx writer: Simplified abstractNumId numbering.

Instead of sequential numbering, we assign numbers based on the
list marker styles.  This simplifies some of the code and should
make it easier to modify numbering in the future.
Docx writer: Changed abstractNumId numbering scheme.

Now the minimum id used by pandoc is 990.  All ids start with "99".
This gives some room for a reference.docx to define numbering styles.
Note:  this is not yet possible, since pandoc generates numbering.xml
entirely on its own.
Docx writer:  pandoc uses only numIds >= 1000 for lists.

This opens up the possiblity (with further code changes) of
preserving some numbering from the reference.docx (e.g. header
numbering.)  See #1305.
Docx writer: Combine reference.docx numbering with pandoc's.

This should have fixed #1305, allowing the reference.docx to define
section numbering, but it doesn't.  Now the headings appear with proper
indentation, but the numbers don't appear.  Unclear why.  styles.xml and
numbering.xml basically match the docx which has the expected result.
Docx writer: Section numbering carries over from reference.docx.

Closes #1305.