Add `content` property to image token

close https://github.com/markdown-it/markdown-it/issues/184

lib/rules_inline/image.js

@@ -12,6 +12,7 @@ var isSpace              = require('../common/utils').isSpace;
 module.exports = function image(state, silent) {
   var attrs,
       code,
+      content,
       label,
       labelEnd,
       labelStart,
@@ -136,8 +137,10 @@ module.exports = function image(state, silent) {
   // so all that's left to do is to call tokenizer.
   //
   if (!silent) {
+    content = state.src.slice(labelStart, labelEnd);
+
     state.md.inline.parse(
-      state.src.slice(labelStart, labelEnd),
+      content,
       state.md,
       state.env,
       tokens = []
@@ -146,6 +149,8 @@ module.exports = function image(state, silent) {
     token          = state.push('image', 'img', 0);
     token.attrs    = attrs = [ [ 'src', href ], [ 'alt', '' ] ];
     token.children = tokens;
+    token.content  = content;
+
     if (title) {
       attrs.push([ 'title', title ]);
     }