Change Cache Duration from Forever to a Configured Value

[alxndrmlr]

We got burned by the default setting where toHtml will add the results to the cache forever.

Specifically we were using this package to convert to markdown the streamed chunks from OpenAI and showing them to the User in the UI. Our dynamodb quick exploded in size since we'd have N prompts * M chunks of unique cache results being written to our cache.

While it probably makes sense for legacy reasons to keep the default of 'cache_store' => null in the config, I'd suggest that we it would be safer for folks to have the results cached for days instead of years re: rememberForever.

Simple change would be 'cache_duration' => null in the config which would rememberForever but then let the user set their own TTL

[alxndrmlr]

FYI I opened #76 to address this