Add support for fragment parsing templates

I have a different workflow for using Roadie where I'd like to
precompile all of my templates. Roadie almost supports this out of the
box, however using `Nokogiri.parse` method will wrap any template in
`<html>` tags if it doesn't already contain it. This doesn't work for me
precompiling any template file (other than layouts).

This pull request simply provides a simple way to pass document options
to the Inliner. The only document option currently supported is
`:fragment` which switches the parser to use `Nokogiri.fragment` instead of
`Nokogiri.parse`.

It also skips the `add_missing_structure` method in the Inliner if the
html node is not present in the parsed document.

Feedback is more than welcome, I want to get this "right" for roadie. I
also have a rake task that I've written that does the precompile for a
given set of files. I was originally going to include that in this
request but decided against it since it may make too many assumptions
about a given application structure.