Allow options to be passed to chokidar

[rdallasgray]

Allow the user to pass options to chokidar:

var w = watchify(b, {watchOptions: {usePolling: true}});

The example given illustrates my use case -- when developing using Vagrant, filesystem events raised on the host don't propagate to the guest, so I'm forced to use polling.

[rdallasgray]

(Apologies, should have read the existing PRs more carefully).

[zertosh]

Further discussion in #162

[zertosh]

Just published 3.0.0 (see #170), it has the ability to use polling.