Remove crumb references and instead document that context properties get added as query params

CSRF would be done by sending in the token to the `options.context` constructor param. These parameters are automatically added to the query params for XHR requests so that the server can use them accordingly.

CSRF token checking would happen in a middleware in front of fetchr's middleware: `app.use('/myCustomAPIEndpoint', csrf(), Fetcher.middleware());` It's up to the application to decide which middleware to use in this case.