Add htpasswd-style static file auth module

Can be used via `-auth.url=file://`. Only supports bcrypt password
hashes ($2y). Use e.g. `htpasswd -c -BC 14 <filename> <user>` to create
a file. Documentation forthcoming.

auth/url.go

@@ -18,6 +18,12 @@ func NewFromURL(authURL string) (AuthProvider, error) {
 		return NewIMAP(u.Host, true), nil
 	case "pam":
 		return NewPAM()
+	case "file":
+		path := u.Path
+		if u.Host != "" {
+			path = u.Host + path
+		}
+		return NewHtpasswd(path)
 	case "null":
 		return NewNull()
 	default: