Add ability to invalidate require cache on rewired file

[mboudreau]

While running these tests, I'm trying to clear the require cache for every test to try to 'sandbox' things and not things set outside the export interfere when they're in a different state.

I would like a way to be able to force invalidate the cache when trying to rewire. Currently I have a workaround of deleting it manually before the test starts, but I think it would be neat if I could simply do rewire('./file', true) to force it to invalidate and get the new rewired file. I've modified it on my fork showing what I mean which could be cleaned up or adding at a better place.

[JvJefke]

Rewire will always return a new instance of the module. See the Caveats section in the readme.

[mboudreau]

@JvJefke but sub-dependencies aren't, which kinds of defeat the purpose.