Add possibility to cache internal results

[Viatorus]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/LokiJS-Forge/LokiDB/blob/master/CONTRIBUTING.md#question

Current behavior

Internal token results are not cached. So an equal or similar full-text search query cannot be optimized from the previous query.

Expected behavior

• Cache token results of a full-text search query.
• Make option optional.
• Clear cache if any document is dirty.
• Maybe LokiDB query results can be cached too.

What is the motivation/use case for changing the behavior?

Better performance for search bars, if only some words have been changed.