Feature request: Add time restriction

[christianbltr]

In some use cases it would be helpful to be able restrict the indexed elements to those which fall into a certain time range based on the current date similar to the "timeRestriction" setting in news:

https://docs.typo3.org/p/georgringer/news/main/en-us/Reference/TypoScript/PluginSettings.html#time-limit-low-timerestriction

An idea for the implementation would be:

• Add Two date fields in the indexer configuration ("timeRestrictionLow", "timeRestrictionHigh")
• Adopt each indexer to check if the date of an indexed element falls into the time restriction
• Update documentation
• Update ke_search_hooks to show how to use this new option

see also: #177