[loader] Add "use strict" or document node 6 requirement

[pward123]

I get the following error when using the loader in webpack 2 with node 4.6.1:

Module build failed: SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode

This can be fixed by adding "use strict"; to the top of loader.js or switching to node 6. I'm not sure if you want to add strict mode or document the requirement.

[jnwng]

there's nothing in the webpack loader that would necessitate a hard dependency on node 6, so i would hesitate to suggest enforcing that.

there's a couple of solutions here:

1. compile down the loader code ala babel-loader which would future-proof this particular module
2. just change the let statement to a var statement or add the use strict; directive you mentioned.

while i'd prefer that we do the former (compiling the loader), i'd be happy to accept a PR with the latter (adding use strict; or changing the let statement) in the meantime.

[jnwng]

linking with #32 as i suspect this will be something to address when the webpack loader is extracted

[jnwng]

addressed by #41. @pward123 unfortunately didn't see that you had created this in your fork, in the future you can send a pull request so that we can pull your code back into this repository.