Closes #548

Fixes #612.

stdbuf is not present on all systems. nice is not builtin and is POSIX.

The effect of the bug was that «-:» was considered an option taking an argument
(see the parsing of the associative array in lines 692-693).

As to preventing recurrence, add a warning comment.  We _could_ change
the separator from colon to something else, but colon is idiomatic for
this use (see, e.g., passwd(5)), and the problem will be unlikely to
recur if and when we add a third field to the assoc's values.  (For
example, jexec(1), chroot(1), and even ssh(1) would benefit from a third
field saying how many positional arguments to skip before the positional
argument that's to be the command word — though in the last two cases,
specifying an "inner" command is optional.)

As of this writing, it seems that the 'type' builtin poisons the commands hash
in a way that breaks AUTO_CD to absolute paths that don't end with a slash.

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