* Makes provided bazel rules look up @emsdk workspace instead of local workspace
* Uses system-specific emscripten binaries instead of defaulting to linux
* Provides macros for loading emsdk dependencies (nodejs and emscripten binaries)
* Unhardcodes paths in bazel rules and .sh wrappers
* `update_bazel_workspace.sh` now updates `revisions.bzl`
* `emscripten_deps()` can be fed with specific emscripten version
* Adds external usage test
Addresses #650 and #696
* allow the bazel toolchain to output html files
* allow for cc_binary rule names to end in .js
* fix python name
* continue to call emcc instead of em++ for now
* small cleanup
Co-authored-by: Mitch Foley <mitchfoley@chromium.org>
