Added markdown reference recognition and output to JSON.

Added markdown reference recognition and output to JSON.

BUG=
R=efortuna@google.com

Committed: https://code.google.com/p/dart/source/detail?r=24009

[Tate Mandel, 2013-06-13 19:59:10]

These are my last changes to docgen before Janice takes it over, so it isn't a
very large change. It just adds the structure for resolving reference links and
allows for output to json as well as yaml.

[Emily Fortuna, 2013-06-13 20:11:44]

https://codereview.chromium.org/17000002/diff/1/pkg/docgen/bin/docgen.dart
File pkg/docgen/bin/docgen.dart (right):

https://codereview.chromium.org/17000002/diff/1/pkg/docgen/bin/docgen.dart#ne...
pkg/docgen/bin/docgen.dart:1: /**
add copyright notice at the top of this file, please. :-)

[Tate Mandel, 2013-06-13 20:24:56]

https://codereview.chromium.org/17000002/diff/1/pkg/docgen/bin/docgen.dart
File pkg/docgen/bin/docgen.dart (right):

https://codereview.chromium.org/17000002/diff/1/pkg/docgen/bin/docgen.dart#ne...
pkg/docgen/bin/docgen.dart:1: /**
On 2013/06/13 20:11:44, Emily Fortuna wrote:
> add copyright notice at the top of this file, please. :-)

Done.

[Emily Fortuna, 2013-06-13 20:28:53]

lgtm

[Tate Mandel, 2013-06-13 20:31:19]

Committed patchset #2 manually as r24009 (presubmit successful).