add support for deferred initialization and blueprint routes in SitemapperExtend class

[renph]

Thank you @h-janes for creating this amazing extension.

This PR

• introduced a deferred initialization to the extension, check init_app
• added support for blueprint routes
  • no need to explicitely call sitemapper.add_endpoint for large or complex project
  • endpoint name can be found by traversing app.view_functions dict

Note that blueprints must be registered before initializing sitemapper extension

All changes are made to SitemapperExtended class, we can merge these into Sitemapper class later

[h-janes]

Thank you for these important improvements! I'm glad you like the extension :)

I will work on incorporating your code into the Sitemapper class and the readme when I get a chance.