Proposal: Add --filter flag to logs command with before and since filter.

This is a follow on from #9338.

This patch adds a flag --filter to the logs command. It includes two filters, `before` and `since` which will show logs since a time point or before or both if combined.

`yyyy-dd-mm hh:mm`: logs from that day, and time onwards
`yyyy-dd-mm`: logs from midnight on that day and onwards
`hh:mm`: logs from that time today and onwards.

Caveats:

* I moved the filter flag processing code into a method (Args.GetAsJSON) in filters/parse/parse.go.    There may be other instances of the orignal code that need updating to this.
* Shortcode flag for the filter is `r`.  `f` was taken by `follow`.