Flask-sitemapper

This release adds further support for dynamic routes, resolving issue 12

The `url_variables` argument can now be either a `dict` (as before) or a `Callable` which returns a `dict`. If a `Callable` is provided, this will be run on each call to `sitemapper.generate()` and will run inside the app content.

This allows for sitemaps to update while the site is live, for example adding a new user page if a new user signs up. See relevant documentation at https://github.com/h-janes/flask-sitemapper/wiki/Usage#dynamic-routes

Adds support for different lastmod, changefreq, and priority arguments per URL variable in dynamic routes. (Issue 9)

Fixes issue with dynamic routes defined before initialisation.

* Adds support for dynamic Flask routes (see wiki)
* Now stores sitemap XML after the first request instead of generating it on each request
* Tests have been improved
* Documentation has been updated