proposal: enable code coverage

[deer]

I created a branch to enable coverage in my fork and added this commit: deer@c92e910

I sort of did it wrong, because each test is uploading coverage which is a bit wasteful. But the results were interesting nonetheless: https://coveralls.io/github/deer/fresh. 75% isn't exactly great. There's a bit of noise here since it's including the tests folder itself, which is a bit silly. Two key parts stand out:

• 78.86 context.ts
• 67.9 render.ts

The biggest thing we could do to increase our coverage is to finally start testing all the CSP stuff, which I highlighted here.