[cache] Allow caching behaviour to be configurable

[JorgenEvens]

This PR makes the default caching behavior configurable by exposing the default implementation as a class which takes sourceTTL and sourceSize as options.

import Avatar, { Cache, ConfigProvider } from 'react-avatar';

const cache = new Cache({

    // Keep cached source failures for up to 7 days
    sourceTTL: 7 * 24 * 3600 * 1000,

    // Keep a maximum of 20 entries in the source cache
    sourceSize: 20
});

// Apply cache globally
<ConfigProvider cache={cache}>
    <YourApp>
        ...
        <Avatar name="Wim Mostmans" />
        ...
    </YourApp>
</ConfigProvider>

// For specific instances
<Avatar name="Wim Mostmans" cache={cache} />

Resolves #161