Allow numbered lists starting from zero

Fixes: https://github.com/markdown-it/markdown-it/issues/129

lib/rules_block/list.js

@@ -128,7 +128,7 @@ module.exports = function list(state, startLine, endLine, silent) {
     markerValue = Number(state.src.substr(start, posAfterMarker - start - 1));
 
     token       = state.push('ordered_list_open', 'ol', 1);
-    if (markerValue > 1) {
+    if (markerValue !== 1) {
       token.attrs = [ [ 'start', markerValue ] ];
     }
 

test/fixtures/markdown-it/commonmark_extras.txt

@@ -108,6 +108,19 @@ test
 </code></pre>
 .
 
+List starting with zero:
+.
+0. foo
+1. bar
+2. baz
+.
+<ol start="0">
+<li>foo</li>
+<li>bar</li>
+<li>baz</li>
+</ol>
+.
+
 
 Coverage. Directive can terminate paragraph.
 .