Add "template" argument permitting modifications to the page wrapper component

[hydrosquall]

A feature that I occasionally used in styleguidist when documenting components in an existing codebase was the ability to add some elements or imports to the shared HTML that wrapped all the fixtures.

Arguably these components could be rewritten to not depend on global state, but having this option eased the migration path when integrating certain legacy components. I was wondering if people would be interested in having that feature in cosmos too.

https://github.com/styleguidist/react-styleguidist/blob/master/docs/Configuration.md#template