Add `time.Time` type support to `where` tpl func

Add `time.Time` type support to `where` tpl func

`where` tpl function doesn't support `time.Time` type so if people want
to compare such values, it's required that these values are converted
into `int` and compare them.

This improves it. If `time.Time` values are passed to `where`, it
converts them into `int` internally, compares them and returns the
result.

See also
http://discuss.gohugo.io/t/future-posts-and-past-posts/1229/3