Prepare for npm publish, Upgrade rules_nodejs to 2.0.0 (bazelbuild#42)

* Add npm package build target

* Make postcss@^7.0.7 a peerDependency

* Use BEGIN-INTERNAL/END-INTERNAL to expose root BUILD in npm package

* Document installation via npm or repository rules

* rules_nodejs 2.0.0

* Switch ctx.actions.run to run_node as required for rules_nodejs 2.0

* Move defs.bzl to index.bzl, add absolute paths that get substituted

* Specify .bazelversion of 3.0.0

* Update rules_sass

* Create DEVELOPING.md

* Mark as 0.5.0