Filter by URL regex

[pl4nty]

I'd like to download only certain resources so it runs faster. Apps with sourcemaps are a good example - I only want .ts files, and I don't want to wait for many .js files to compress and download. A regex on the URL seems ideal because it covers plenty of usecases:

• One or more file extensions
• All files in a folder/subfolders
• All files from a host
• Include or exclude

I'm happy to send a PR for this.