LocalDownloadStrategy for pre-downloaded archives

This strategy 'downloads' unaltered, already-downloaded archives from a local directory defined by the `HOMEBREW_LOCAL_DOWNLOADS` environment variable. Helpful for installing SDKs and other proprietary software that require the user to agree to licenses online first, i.e. no direct download link. It's triggered by a custom `local://` protocol.

Example:

```ruby
class MrsidSdk < Formula
  homepage "https://www.lizardtech.com/developer/"
  url "local://MrSID_DSDK-9.0.0.3864-darwin12.universal.gccA42.tar.gz"
  sha1 "8a693cc71dbb8638f34e35efb8086f29b08fa764"
  version "9.0.0.3864"
  ...
end
```

This resolves to following URL for curl, assuming `HOMEBREW_LOCAL_DOWNLOADS=/Users/Me/SDKs/Archive`:

```
file:///Users/Me/SDKs/Archive/MrSID_DSDK-9.0.0.3864-darwin12.universal.gccA42.tar.gz
```

The interruption to a general `brew install` workflow is similar to unmet Python module dependencies that need installed.