Allow passing isRequire detector as an option

I develop async loading mechanism on top of browserify modules and I'm using `require.async(id, callback)` syntax as async dependency declarations. That patch would allow me to reuse `detective` code to extract such dependencies.

It's not desirable for me to use `word` option cause that way I would need to introduce new "global" and thus become incompatibe with `browser-pack` which only passes `module`, `require` and `exports` to a module.