Allow the use of multiple site webfile resource bundles

As a website visitor I would like HippoCMS to allow multiple site webfile resource bundles so when changes are made to front-end assets I do not have to re-download all the files from the servers (just the ones on the site and / or type) that have changed.

Scenario
We want to run all our company sites through one Hippo instance however if we do this and make a small change to SITE A's css the visitors will have to re-download all the files on SITE A and then if they visit SITE B will also have to re-download all the files there too. This is very annoying when trying to optimise our website performance for mobiles and devices with lower bandwidth.

If we were allowed multiple site webfile resource bundles we could simply only increase the cachebust values for the file groups / sites we have changed.

Acceptance Criteria
*Mutliple webfile resource bundles can be defined in the CONSOLE
*Resource bundles have independant cachebust values and are only increased if their content changes
*Different bundles can be accessed by specifying them in the ftl macro