Feat(source): Support specifying more time intervals(like 1d, 1w, 1y) for the "ignoreOlder" config of file source (#153)

* Feat(source): use model.duration(from github.com/prometheus/common/model) to replace time.duration to support "1d"、"1w" and "1y" time internal for the "ignoreOlder" config of file source
Refactor: sort import(use uber guide: https://github.com/uber-go/guide/blob/master/style.md#import-group-ordering)