Polybuild doesn't allow vulcanize exclusions

[otter-computer]

As far as I can tell there is no way to exclude certain files from Polybuild when using it with Gulp. This becomes an issue when using it in a Cordova application as cordova.js is not injected until the end of the build process.

Please tell me if there is a solution to this, otherwise wait until the project is more mature before including it in the Polymer Starter kit.

[ebidel]

Polybuild is meant as a one-stop shop that combines the defaults of a few tools. If you want more control, I suggest using the tools separately.

cc @addyosmani for PSK usage.

[otter-computer]

Just because it combines them, doesn't mean you should lose control of them though, no?

[indolering]

@ThomasShaped A PR might be more convincing : )