support app node_modules

[ahdinosaur]

we use the src/node_modules pattern in our apps (e.g. here and here), so the default ignore of **/node_modules/** doesn't work for us.

previous discussion in #35.

possible solutions already brought up:

1. "we could ignore node_modules if a package.json is present in the same directory"
2. implement a way to un-ignore a directory

1 makes a lot of sense, i don't see how it wouldn't work as the src/node_modules pattern as i understand it doesn't involve a src/package.json. keen to hear what people think, am happy to implement a PR for this.

[feross]

Yeah, I actually think 1 would work. Happy to consider a PR for it.