Extend loader context (to support less-loader)

[Mesoptier]

The less-loader package uses the fs and loadModule properties on the loader context, but those were not provided by this package. So this PR adds them in.

A few notes:

• The fs property is simply the imported NodeJS fs module. So if the user has configured a different filesystem using Webpack's inputFileSystem option this will not be forwarded into the workers. This might cause unexpected issues, but I don't directly see a nice way around this issue.
• The actual loadModule function returns (besides source and sourceMap, which are strings) the module, which is an instance of NormalModule. This class cannot be trivially serialized, so I pass undefined instead. Again, this might cause unexpected issues.

Additionally, it looks like future less-loader versions will use getResolve instead of resolve just like sass-loader. So it would probably be a good idea to add support for that as well.

[jsf-clabot]

All committers have signed the CLA.

[alexander-akait]

Sorry for delay and thanks for the PR, let's improve serialization in other PRs