Don't require a space after colon in link refs

Fixes https://github.com/dart-lang/markdown/issues/176

pkgs/markdown/lib/src/block_parser.dart

diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index cdeb1ab..0c5be70 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -957,7 +957,7 @@
         // Leading indentation.
         r'''^[ ]{0,3}'''
         // Reference id in brackets, and URL.
-        r'''\[([^\]]+)\]:\s+(?:<(\S+)>|(\S+))\s*'''
+        r'''\[([^\]]+)\]:\s*(?:<(\S+)>|(\S+))\s*'''
         // Title in double or single quotes, or parens.
         r'''("[^"]+"|'[^']+'|\([^)]+\)|)\s*$''',
         multiLine: true);

pkgs/markdown/test/original/reference_links.unit

diff --git a/pkgs/markdown/test/original/reference_links.unit b/pkgs/markdown/test/original/reference_links.unit
index 02a1c56..6b22081 100644
--- a/pkgs/markdown/test/original/reference_links.unit
+++ b/pkgs/markdown/test/original/reference_links.unit
@@ -82,3 +82,10 @@
 <<<
 <blockquote>
 <p>links <a href="http://foo.com">are</a> awesome</p></blockquote>
+>>> reference link regression for github.com/dart-lang/markdown/issues/176
+[![Coverage Status][coverage_status]][coverage_page]
+
+[coverage_page]:https://coveralls.io/github/yeradis/stay_points.dart?branch=master
+[coverage_status]: https://coveralls.io/repos/github/yeradis/stay_points.dart/badge.svg?branch=master
+<<<
+<p><a href="https://coveralls.io/github/yeradis/stay_points.dart?branch=master"><img alt="Coverage Status" src="https://coveralls.io/repos/github/yeradis/stay_points.dart/badge.svg?branch=master" /></a></p>
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