facho: clip.py se adiciona nuevo comand 'sign-xml'.

* facho/cli.py: adiciona nuevo comando 'sign-xml' para firmar
directamente un xml.
* facho/fe/fe.py (DianXMLExtensionSigner.sign_xml_string): Nuevo metodo.

FossilOrigin-Name: 61920c40da14a134de6392845b3e4d98ad2b1b683093038d6161c147669127e9

facho/facho.py

@@ -98,6 +98,7 @@ class LXMLBuilder:
     def set_attribute(self,  elem, key, value):
         elem.attrib[key] = value
 
+    @classmethod
     def tostring(self, elem, **attrs):
         attrs['pretty_print'] = attrs.pop('pretty_print', False)
         attrs['encoding'] = attrs.pop('encoding', 'UTF-8')