chore(docs): Clean up verbage in README.md
refactor: Eliminate middleware global variables. All access is through instance variables.

* The `setup` function returns a middleware instance where all functions and variables can be accessed.
* This change is to facilitate testing and setting up multiple instances of connect-fonts.
Bump the version to 2.0.0 because of breaking API changes.

BREAKING CHANGES:

* All function and variable access must occur through the middleware instance returned by the `setup` function.
fix: Expose the `generate_css` function on middleware.
Merge branch 'instances'