M1456: Implement MIME Sniffing

Bring on the review!

To show mime-sniffing feature:

Open gif in servo.
	```./mach run tests/content/parsable_mime/image/gif/test87a```
Copy image outside of version control
	```cp tests/content/parsable_mime/image/gif/test87a ..```
Checkout changeset without feature.
	```git checkout 0d2251510fb9ad8f4974c99cadafbd1a9a81e30f```
Attempt to open gif in servo.
	```./mach run ../test87a```