add optional navigator.sendBeacon transport

[tdumitrescu]

fixes #184

Several ways to use it:

// for an individual track() call
mixpanel.track('my event', {my: 'props'}, {transport: 'sendBeacon'});

// turn on for every Mixpanel call when page is unloading
// (you would use this to use sendBeacon for everything, including
// mixpanel.people calls)
window.addEventListener(`unload`, function() {
  mixpanel.set_config({api_transport: 'sendBeacon'});
  mixpanel.track('my event');
  mixpanel.people.set({foo: 'bar'});
});

// initialize for all tracking; not recommended as it will prevent any
// request-retry facilities
mixpanel.init('my token', {api_transport: 'sendBeacon'});
mixpanel.track('my event');

[evnp]

I'm confused by this, missing how it modifies options. Can you add a clarifying comment if it's intentional?

[tdumitrescu]

Will do - it's because track() is a public API and so "transport" shouldn't be minified, whereas _send_request() is internal and so its options.transport can be minified however closure compiler wants.

[evnp]

that makes a strange amount of sense, thanks for the explanation

[evnp]

I'd personally find the code below more readable if I saw navigator.sendBeacon, provides more context even though it's more verbose and you wouldn't need this bind iiuc. Nbd if you prefer it this way.

[evnp]

just realized you probably need this to avoid the minifier messing with 'sendBeacon'; disregard my comment

[tdumitrescu]

Another size optimization, like we do throughout this SDK. It lets closure compiler compress this down by referring to a minified var rather than injecting "navigator.sendBeacon" every time it's used. The minified version of this code is:

U=I.sendBeacon;U&&(U=c.bind(U,I));

so the code below can just refer to U