Fixes "alt" tag not showing, when `images` option is used

[benwinding]

Problem

When using the images: config option, the alt tag doesn't get added to the attributes.

Steps to Reproduce

• Set images in hexo _config.yml file

  markdown:
  images:
  lazyload: true // Need at least one option, this is just an example

• Note the generated <img .../> tags in HTML don't have any alt attributes

Fix

Use the token.content which contains the "alt text" from the markdown image.

Example markdown image

What's available in the context during debugging

Note: I'm not sure if it would be better to fix this up the call-chain :shrug:

• Fixes #205