Do nothing if the target filesystem is not the default

[jonathanperret]

Fix #7 by detecting a non-standard output file system and bailing out.

[jonathanperret]

A more flexible fix would use methods on the provided compiler.outputFileSystem to do the work, but that would require reimplementing recursive-readdir-sync which is hard-wired to fs.

[jonathanperret]

Scratch that, NodeOutputFileSystem does not expose the necessary methods: https://github.com/webpack/webpack/blob/master/lib/node/NodeWatchFileSystem.js

[tekacs]

Any chance this can be merged and the version bumped (here and on NPM)? @gpbl