Allow template arguments to be documented

[ssokolow]

Currently, if I attempt to attach a doc comment to a template argument, like this:

        /// Used for CSS/JS cache-busting
        build_timestamp: &'static str,

...I get the following error:

error: expected identifier
  --> src/templates.rs:58:9
   |
58 |         /// Used for CSS/JS cache-busting
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm left having to manually add an Arguments section containing a bulleted list to the documentation block for the template as a whole, resulting in this needlessly redundant and typo-prone output:

[utkarshkukreti]

I've implemented this in https://github.com/utkarshkukreti/markup.rs/commit/45327272008c74090fa701ee26ff52d55fca9301, please let me know if it works for you.

[ssokolow]

Looks good. :)