Skip non-directories in the domains/ directory

This prevents chasquid from attempting to look for certs under a
non-directory, e.g. `/etc/chasquid/domains/.gitignore/certs`.

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Adjusted commit message, applied `go fmt`.

chasquid.go

@@ -109,6 +109,10 @@ func main() {
 		if err != nil {
 			log.Fatalf("Invalid name %+q: %v", info.Name(), err)
 		}
+		if info.Type().IsRegular() {
+			// Ignore regular files, we only care about directories.
+			continue
+		}
 		dir := filepath.Join("domains", info.Name())
 		loadDomain(domain, dir, s)
 	}