Make receiver hostname configurable via --receiver-hostname flag

Remove the package-level global hostname from parser.go.

Adds a log warning when the last Received hop doesn't match the
expected receiver hostname.

Bug: https://github.com/happyDomain/happydeliver/issues/11

pkg/analyzer/analyzer.go

@@ -41,6 +41,7 @@ type EmailAnalyzer struct {
 // NewEmailAnalyzer creates a new email analyzer with the given configuration
 func NewEmailAnalyzer(cfg *config.Config) *EmailAnalyzer {
 	generator := NewReportGenerator(
+		cfg.Email.ReceiverHostname,
 		cfg.Analysis.DNSTimeout,
 		cfg.Analysis.HTTPTimeout,
 		cfg.Analysis.RBLs,