Add link to maizzle serve UI to preview plaintext templates

[wzrdjstr]

It would be nice if we could switch between HTML and plaintext previews using maizzle serve UI.

For example:

A [text] link could be added beside each template on the the maizzle serve index page which serves a text/plain response containing the plaintext version of the template. Users could then preview HTML and plaintext versions of their templates in different tabs.

[cossssmin]

I'll add it to the backlog for Maizzle 6, today's dev server UI is very limited unfortunately.