common: replace windows file URL backslash with forward slash

/cc @jasonberanek - Just adding this as well because I see this being
common as well.

common/config.go

@@ -78,6 +78,11 @@ func DownloadableURL(original string) (string, error) {
 		// since net/url turns "C:/" into "/C:/"
 		if runtime.GOOS == "windows" && url.Path[0] == '/' {
 			url.Path = url.Path[1:len(url.Path)]
+
+			// Also replace all backslashes with forwardslashes since Windows
+			// users are likely to do this but the URL should actually only
+			// contain forward slashes.
+			url.Path = strings.Replace(url.Path, `\`, `/`, -1)
 		}
 
 		if _, err := os.Stat(url.Path); err != nil {