README update (#504)

* v2.x readme overhaul with additions discussed in #448

added "comments" (TODO link references) for changes suggested but not yet implemented for future discussion/prs
    clarified "native stream" to be "native Node streams"
    adjusted all uses of http to https to encourage secure protocol usage
    adjusted whatwg to proper case, WHATWG
    made code block tags consistent as `js` instead of `javascript`
    uppercased all method option values (post vs POST)
    added spec-compliant node to the `response.ok` api section

* fix left over cruft, inconsistent hierarchy