Refactor http resolver into separate npm package

[jamesramsay]

The HTTP resolver has heavier dependencies (e.g. got) than the rest of hercule. For applications using hercule in the browser, the ability BYO HTTP library is appealing so that I can use reuse axios or some other heavier library I may already be using, or roll my own tiny lightweight solution.

Splitting out a new a new package could easily increase the maintenance burden of this project and make it hard for new resolvers to be contributed. Using a monorepo may be a solution to this (see http://alexwheeler.io/ruby/git/2017/09/16/monorepos.html).