Add "auto" mode that filters commits heuristically

[niosHD]

Large repositories, like the Linux kernel, often contain commits with bogus commit date (e.g., 1970-01-01 01:00:01 +0100, 2085-06-18 15:57:19 +0000, ...). At the moment these commits can be omitted using the --no-future and --max-age=<days> arguments. However, having an (opt-in) "automatic" mode would be nice.