Adds makeRequest() method to workbox.strategies

[jeffposnick]

R: @gauntface @philipwalton

Fixes #1395

This allows any of the workbox.strategies.* classes to be called manually, outside of a Route context.

The JSDocs in this PR refer to https://developers.google.com/web/tools/workbox/guides/advanced-recipes#make-requests, which I'll write following this PR.

[gauntface]

Is there any value in keeping the handle() method with this change?

[jeffposnick]

Removing .handle() entirely would be a breaking change to the interface. While I doubt there are too many folks calling .handle() directly, we'd have to make some changes to the workbox-routing to go along with removing .handle(), and that might be more likely to break existing assumptions.

So I'd rather punt on that until the next round of discussions about breaking interface changes.