feat(localforage): add support for custom drivers

[thgreasi]

Resolves #267

This is a simple implementation, not guessing the use of requirejs, browserify or anything else.
The developer uses an async call like localforage.defineDriver(dummyStorageDriver); and passes a "driver definition object" that should look like

{
    _driver: 'customDriverUniqueName',
    _initStorage: function(options) { },
    clear: function(callback) { },
    getItem: function(key, callback) { },
    key: function(n, callback) { },
    keys: function(callback) { },
    length: function(callback) { },
    removeItem: function(key, callback) { },
    setItem: function(key, value, callback) { }
}

It also may contain a _support property that is either boolean (true/false) or it returns a Promise that resolves to a boolean value. If _support is omitted, then true is used.

For a brief how-to take a look at test/dummyStorageDriver.js and test/test.customdriver.js.

PS: The async nature of the defineDriver method may also prove useful for a future implementation that utilizes requirejs/browserify to load the driver, as presented in this comment of #267.
PS2: Also, my naming may be bad.

[tofumatt]

Whoa! This is awesome and unexpected ^_^ 👍

I'll look through now, but I like the idea.

[tofumatt]

Looking through the code, it looks like each instance of localForage has the defineDriver() method and can load a custom driver object. Does this allow all instances of localForage access to the custom driver, or only the instance that calls defineDriver()?

[thgreasi]

Driver definitions are available for all instances to be used, since the CustomDrivers object (that holds the custom driver definitions) is inside the closure created by the localforage.js IIFE.

[tofumatt]

Awesome. Okay cool. I think there are a few nitpicks but I'll clean them up and merge this in. Awesome stuff!

[tofumatt]

I'll fix this and a few whitespace things before merging, but it's "compliant" 😄

[thgreasi]

It looks like, that besides naming I also have typo issues.
This should also be changed in src/localforage.js test.customdriver.js.

[tofumatt]

Totally awesome; thanks so much 👍

I'm drafting up a release of 1.1 to add this in now :-)

[jprichardson]

Love this, thank you so much!