Allow rack/mount cascading to be skipped on header

When using header versioning with the :strict policy, Grape currently
returns the appropriate error (406 Not Acceptable) but it also returns a
X-Cascade header indicating to rack it should keep looking for
other routes.

While this behavior is expected for various applications, some may need
to skip the cascading and explicitly return the error to the requester.

For that, a :cascade option was added to the version method. The default
value is true (keeping backwards compat), meaning that 406 errors shall
be raised along with X-Cascade headers, just like before. Though, if set
to false, it 406 errors shall be raised with no X-Cascade headers,
therefore telling the router it should not lookup other routes.
Minor README edits for cascade.