Implement getResolve

[qnighy]

Fixes #79 by implementing getResolve. getResolve is a generalization of resolve where resolver.resolve(context, request, callback) is equivalent to resolver.getResolve(null)(context, request, callback). Differences:

• It takes options.
• It's a curried function so getResolve itself returns a function. I guess this is meant to cache options. However options is also cached in the later stage of the resolver, so I decided not to implement caching in the thread-loader side.
• It returns promises if callback is omitted.

Therefore message-wise we can extend the existing resolve message to allow options payload and handle them in the worker pool. In the worker pool, we delegate resolve message to getResolve if options is specified for compatibility with resolvers without getResolve (although I'm not sure there's any). Currying and promise generation are handled on the worker side.

[jsf-clabot]

All committers have signed the CLA.

[alexander-akait]

Good job, but we need tests for this

[qnighy]

Could you provide some hints on testing? I looked at a similar PR #88 and there are no tests. At a glance, this library doesn't seem to contain any tests directly calling functions like resolve or emitWarning.

[alexander-akait]

@qnighy You can create test with sass-loader or less-loader and with @import, they use getResolver

[qnighy]

Thanks! I'll take a shot.

[qnighy]

Added a test that actually runs webpack using the Node interface. I wanted to specify ./src/index.js as an entrypoint but Node rejects it saying the extension must be .mjs so I resorted to ./dist/index.js instead.

[alexander-akait]

Good job, thanks!