Allow site to ask UpUp if offline mode is available

[TalAter]

An idea:

UpUp.isAvailable()
Returns 0 if UpUp requirements aren't supported by current page (e.g. ServiceWorkers, HTTPS, etc.)
Returns 1 if UpUp supported.
Returns 2 if UpUp supported and cache is filled.

[yankeeinlondon]

I'd suggest there be two callable endpoints:

UpUp.isOffline() - boolean flag indicating online/offline status
UpUp.browserSupported() - boolean flag which indicates whether a browser supports SW

Ideally the online/offline information might also be exposed as an event emitter ...

let foo = UpUp.offline;
foo.on('offline', () => {});
foo.on('online', () => {});

This latter functionality might be more easily addressed once the #42 issue is addressed

Finally, no need for an event emitter for browser support as I think this would always be static (outside of a config change like in Firefox 42 but that's too small a edge case to consider a solution for).

[TalAter]

I'm thinking a more generic function than browserSupported()... maybe something like supported() which could also test for other compatibility issues (e.g. HTTPS)

[yankeeinlondon]

I think those messages serve different audiences ... Meaning the HTTPS issues is more of a developer issue whereas the browser support is more of a dynamic client issue. Still I can see that combining them could make sense from an UpUp perspective. If a developer wants to react to client browser they can pretty easily test for it with window.navigator.serviceWorker.