WebFile redirecter

Allow switching URLs in packages for locally-controlled equivalents in Get-ChocolateyWebFile.
This enables packages to install without direct Internet access, and manual caching.

Enabled via the environment variable "ChocolateyWebFileRedirecterCsv", which should point to a uri or filepath of a CSV file in UTF8 (or ASCII) form containing "url,redirectedUrl" columns and one url mapping per row.