Adds an author to blog posts.

[Beanow]

See #113
This adds an author to the blog page only.
Implementation mostly based on Docsy's version.
https://github.com/google/docsy/blob/master/layouts/blog/content.html

[budparr]

Hey, @Beanow I'm basically fine with this. Using author could be problematic, because some may use author for something like a taxonomy lookup, but I think it should be okay.

However, could you wrap the entire p tag in the with statement?

So, instead of

<p class="tracked">
    {{ with .Params.author }}By <b>{{ . | markdownify }}</b>{{ end}}
</p>

use:

{{ with .Params.author }}
    <p class="tracked">
        By <strong>{{ . | markdownify }}</strong>
     </p>
{{ end}}

[Beanow]

Would have been happy with you making the change, but here you go 👍