support site root paths

Hi,

This evening I tried vulcanizing a number of custom polymer-elements using site root path and I got the same file outputted more or less in a silent fashion. I added a flag called ``webAbsPath`` that allowed me to vulcanize resources referenced with site root path. This seems to relate to https://github.com/Polymer/vulcanize/issues/18.

Can we please add this option?

Thanks very much,

@jongeho1