Allow custom filename for AssetCache

[chriskirknielsen]

This fixes #24, allowing custom filenames for AssetCache via a callback function taking the key and hash as arguments.

Usage:

const cachedApiData = new AssetCache('my-wonderful-api', '.cache', { hashLength: 8, cacheFilename: (key, hash) => `11tycache-${key}-${hash}` });

Might be interesting to explore this to hash the contents instead so the filename is more unique but I think this should cover base cases at the very least.

Additionally, lines 25–27 might be overzealous and could likely be discarded, since the getter does check for length meaning we'd fall back to the default filename. Please edit to your heart's content!

[zachleat]

Merged, thank you! Though we’ll probably rename the option filenameFormat to match the Image utility convention