paste_linter() should ignore URL-like paths for file.path() recommendation by default

[MichaelChirico]

Follow-up to #2074.

Originally posted by @AshesITR in #2074 (comment)

Not all authors will like using file.path() to build URLs, so paste0() may be preferable in such cases. The proposed solution is to skip any paste0() call with a double-slash //, and use an argument for authors who may want to lint such paths anyway.