Add alignment feature to tables

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 162734b..0b03b8a 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -49,7 +49,7 @@
     new RegExp(r'^([ ]{0,3})(\d{1,9})([\.)])(([ \t])([ \t]*)(.*))?$');
 
 /// A line of hyphens separated by at least one pipe.
-final _tablePattern = new RegExp(r'^[ ]{0,3}\|?(\-+\|)+\-*$');
+final _tablePattern = new RegExp(r'^[ ]{0,3}\|?(:?\-+:?\|)+(:?\-+:?)?$');
 
 /// Maintains the internal state needed to parse a series of lines into blocks
 /// of Markdown suitable for further inline parsing.
@@ -727,21 +727,34 @@
   /// * a divider of hyphens and pipes (not rendered)
   /// * many body rows of body cells (`<td>` cells)
   Node parse(BlockParser parser) {
-    var head = new Element('thead', [parseRow(parser, 'th')]);
+    var alignments = parseAlignments(parser.next);
+    var head = new Element('thead', [parseRow(parser, alignments, 'th')]);
 
     // Advance past the divider of hyphens.
     parser.advance();
 
     var rows = <Element>[];
     while (!parser.isDone && !parser.matches(_emptyPattern)) {
-      rows.add(parseRow(parser, 'td'));
+      rows.add(parseRow(parser, alignments, 'td'));
     }
     var body = new Element('tbody', rows);
 
     return new Element('table', [head, body]);
   }
 
-  Node parseRow(BlockParser parser, String cellType) {
+  List<String> parseAlignments(String line) {
+    line = line
+        .replaceFirst(new RegExp(r'^\|'), '')
+        .replaceFirst(new RegExp(r'\|$'), '');
+    return line.split('|').map((column) {
+      if (column.startsWith(':') && column.endsWith(':')) return 'center';
+      if (column.startsWith(':')) return 'left';
+      if (column.endsWith(':')) return 'right';
+      return null;
+    }).toList();
+  }
+
+  Node parseRow(BlockParser parser, List<String> alignments, String cellType) {
     var line = parser.current
         .replaceFirst(new RegExp(r'^\|\s*'), '')
         .replaceFirst(new RegExp(r'\s*\|$'), '');
@@ -767,6 +780,11 @@
     });
     row.add(new Element(cellType, cellContents));
 
+    for (int i = 0; i < row.length && i < alignments.length; i++) {
+      if (alignments[i] == null) continue;
+      row[i].attributes['style'] = 'text-align: ${alignments[i]};';
+    }
+
     return new Element('tr', row);
   }
 }

pkgs/markdown/test/extensions/tables.unit

diff --git a/pkgs/markdown/test/extensions/tables.unit b/pkgs/markdown/test/extensions/tables.unit
index e28def5..e16cde5 100644
--- a/pkgs/markdown/test/extensions/tables.unit
+++ b/pkgs/markdown/test/extensions/tables.unit
@@ -45,3 +45,11 @@
 row with | two cells
 <<<
 <table><thead><tr><th>head</th><th>foo</th><th>bar</th></tr></thead><tbody><tr><td>body</td></tr><tr><td>row with</td><td>two cells</td></tr></tbody></table>
+>>> left, center, and right alignment
+head | cells | here
+:----|:-----:|----:
+body | cells | here
+too | many | cells | here
+
+<<<
+<table><thead><tr><th style="text-align: left;">head</th><th style="text-align: center;">cells</th><th style="text-align: right;">here</th></tr></thead><tbody><tr><td style="text-align: left;">body</td><td style="text-align: center;">cells</td><td style="text-align: right;">here</td></tr><tr><td style="text-align: left;">too</td><td style="text-align: center;">many</td><td style="text-align: right;">cells</td><td>here</td></tr></tbody></table>