Add ID scraper

[chriszs]

Add Idaho scraper.

Steps to test

python -m warn.cli ID

Closes #82

[chriszs]

Deals with gaps caused by merged cells, like this:

By filling down like this:

Notice we're excluding the number of employees column to preserve the merged cell total and avoid double counting.

[chriszs]

This one's ready for review, @palewire.

[palewire]

Thanks for the creative use of pdfplumber here. Big picture it all looks great. I just have a couple small things I've flagged. Let me know what you think.

[chriszs]

Thanks for the suggestions! Refactored to use cache.download (and added a feature to that to pass verify or any other kwarg), added some util functions to make that one if statement test read clearer, integrated your suggestions and cleaned everything up.

[palewire]

Thanks! You're in!