#210 Allow to set binding_classes independently of doc parsing

[Eric van der Vlist]

When I want to create elements matched to a specific class through doc.xml_create_element() on an Amara document generated from scratch and, unfortunately, the only method to do so is through binderytools.setup_binder_() which name suggests that it is not really public and that isn't very user friendly (their parameters have no default values).

I think that we would need a "more public" method to do so and I am also wondering if that's not a limitation that these bindings are global: what if I manipulate two (or more) different documents at the same time and don't want to assign them the same mappings?

Shouldn't these be properties of the documents rather than global variables?

Thanks,

Eric