Scrapers now support --archive, to go back to their earliest year

`inspector.year_range` now takes options and an archive year, so any scraper that passes a year to that function will support an `--archive` flag.

I've updated each scraper to replace its top-of-file comment with an actual variable declaration (e.g. `archive = 2006`), and each call to `inspector.year_range` to pass this variable in.

The `--archive` flag will work in either form of running a scraper:

```bash
./igs --archive
./inspectors/usps.py --archive
```

It's now possible to easily re-run the scrapers over their entire archive, and keep up with improvements to scraping and metadata that affect the entire set.