New Featue : Support rest-docinfo metadata for nikola

Recently nikola started supported variety of metadata formats.

Currently, the posts with the following style of meta-data format say "Can't Parse" 

    :title: My Title
    :slug: my-title
    :date: 2012-09-15 19:52:05 UTC

By supporting https://getnikola.com/handbook.html#rest-docinfo , you may be able to support `pelican` blog platform with blog-admin  