Add Markdown Syntax Highlight

Thanks Daniel for the set -- suggestion.

Fixes #596

This reverts commit ba2d8fcf76.

Breaks 4.3.12 and .11

This would have caught #593.

That might not be idempotent (cf #598).

Required for TAP compliance in case the output ever contains '#' or '\n'.

Stylistic tweaks only; no functional change, no effect on TAP compliance.

Needed for next commit.

Fixes #590.

* i511-bang-pipeline:
  main: Fold '!' handling in to the 'case'.  No functional change.
  Fix indentation.  No functional change.
  main: Fix highlighting of the ! precommand after array assignments. Add tests.
  main: Highlight the ! precommand as an error when not at the start of a pipeline.

Fixes #511.

Fixes highlighting when an unclosed $( ends in whitespace.

Merge remote-tracking branch 'danielsh/i238-named-fd-redirection-v1'

* danielsh/i238-named-fd-redirection-v1:
  'main': Tighten condition.
  noop: Tweak condition at Matthew's suggestion
  'main': Highlight named fd redirections.

Should rule out brace expansions such as '{foo,bar}' and '{10..20}'.

Fixes #238

Closes #226

Fixes #347

Fixes #540 #544 #552 #554 #555

No functional change. Prepares for running a full alias through the
state machine.

Related to https://github.com/zsh-users/zsh-syntax-highlighting/issues/549#issuecomment-431628338 and to #556.

Update the test point for f3410c5862 (#264),
which changed the highlighting of aliases to consider what they resolve to.

Now the test file has both ${aliases[alias1]} and ${functions[alias1]},
the expectation is 'alias', and passes; thus:

Fixes #588.

The test never actually defined a function named 'alias1', not even
when zsh 5.4 warned it about this (see 9523d6d49cb3d4db5bd84c3cec6168a2057fe3ab,
which was wrong and is hereby reverted).

The test that's now failing has been filed as issue #558.

This avoids a bug in zsh 4.3.12 and prior which affects passing
arguments to an anonymous function.

Closes #547.