Added `lists_without_preceding_blankline` extension.

* Added `Ext_lists_without_preceding_blankline` to
  `Extension` in `Options`.  Added this option to
  `githubMarkdownExtensions`.
* Made markdown reader sensitive to this.
* Closes #972.
MedaWiki reader:  Implement some mathjax extensions.

* `:<math>` for display math
* `\(..\)` for inline math
* `\[..\]` for display math

We omit the `$` forms as the heuristics are harder.
HTML5 template: Add meta tag to allow user scaling.

(Erik Evenson)
Partial reorganization of changelog.
Markdown: Allow backtick code blocks not to be preceded by blank line.

Closes #975.
markdown+list_without_preceding_blankline:+Interpret text before list as paragraph.
More work on changelog.
Templates:  more consistent behavior of `$for$`.

When `foo` is not a list, `$for(foo)$...$endfor$` should behave like
$if(foo)$...$endif$.  So if `foo` resolves to "", no output should
be produced.

See pandoc-templates#39.
Added more raw material to changelog.
Made . . . for pause work in all slide show formats except slideous.
More work on changelog.
More work on changelog.
More work on changelog.
`--bibliography` no longer implies `-F pandoc-citeproc`.

This could lead to double filtering if the user specifies `-F`
too.
Markdown:  don't parse citation right after alphanumeric.

An `@` after an alphanumeric is probably an email address.