Potential canonical check?

Firstly great tool, thank you.

I'm currently scanning a few sites for mixed content warnings, another error i've spotted which is hard to catch is that a few pages have had hardcoded canonical urls with `http://` rather than `https://`.

I've got a bash script going through and scanning urls to ensure the canonicals correctly point to `https` but what do you think about baking a warning like that into this script? Something like

`[2015-01-07 12:54:47] MCS.WARNING: `

While it's not strictly a mixed content warning it's something I'm having to catch another way, do you think this check would have any place in this module or would it open a slipperly slope to too many unrelated checks?

The issue names no files, tests, or entry points; begin by reading the scanner's current mixed-content checks and how warnings are emitted. Confirm the project scope for canonical URL validation with maintainers, then define the expected warning behavior and coverage before implementation.

Written by the indexing model from the issue text.