Add support for manually setting a limit on results

[AlexTalks]

• Added a query parameter to allow user to specify max results, rather than a hard limit at 5000, for each index.
• Added support for a default max result limit in the config. If unset, it remains at the former default of 5000 results per index.
• Fixed issue where no results would be returned if the limit was exceeded - now it simply returns the results up to the limit.

Resolves #217