Allow optional requires

I am attempting to use browserify-cdn to bundle the NPM module [cucumber](https://www.npmjs.com/package/cucumber), however I receive the following error:

Error: "browserify exited with code 1"

code: 1
stderr: Error: Cannot find module 'graceful-fs' from '/tmp/cucumber120310-6085-1fs7afr.h95u/node_modules/cucumber/node_modules/mz'
at /home/wzrd/wzrd.in/node_modules/browser-resolve/node_modules/resolve/lib/async.js:46:17
at process (/home/wzrd/wzrd.in/node_modules/browser-resolve/node_modules/resolve/lib/async.js:173:43)
at ondir (/home/wzrd/wzrd.in/node_modules/browser-resolve/node_modules/resolve/lib/async.js:188:17)
at load (/home/wzrd/wzrd.in/node_modules/browser-resolve/node_modules/resolve/lib/async.js:69:43)
at onex (/home/wzrd/wzrd.in/node_modules/browser-resolve/node_modules/resolve/lib/async.js:92:31)
at /home/wzrd/wzrd.in/node_modules/browser-resolve/node_modules/resolve/lib/async.js:22:47
at FSReqWrap.oncomplete (fs.js:153:21)

After further investigation, I identified the following lines of code within a dependency [mz](https://github.com/normalize/mz/blob/master/fs.js#L4):

try {
fs = require('graceful-fs')
} catch(err) {
fs = require('fs')
}

Would it be possible to add the [browserify-optional](https://github.com/devongovett/browserify-optional) transform into browserify-cdn to support this situation? Perhaps there could be a flag to enable this as an optional configuration?

Alternatively the built-in `ignoreMissing` option may work for my use case. Would it be possible to add support to allow this option to be passed into the bundler?

Start by tracing how browserify-cdn invokes the bundler and how bundler options are passed. Review the browserify-optional transform and the built-in ignoreMissing option mentioned in the issue, then determine which approach fits. Done means optional requires like mz's graceful-fs fallback bundle successfully, with the chosen configuration exposed by browserify-cdn.

Written by the indexing model from the issue text.