select/multi_select: support for adding new tags if some of the tags don't exist (fix for issue 51)

[de-novikov]

The pull request fixes the issue described in issue 51:
#51

[jamalex]

Fantastic, thanks for your work on this!

I would suggest a couple of changes:

• It's a bit risky to overwrite the schema entirely, based on the current local value, since many people are running with auto-refreshing turned off, so it may be stale. To be a bit gentler, perhaps consider something like c.set("schema.{}.options".format(prop["id"]), new_options_list) (with new_options_list being the old list with the new item appended).
• It could be valuable (and keep things cleaner) to move a lot of this logic to a separate method, on Collection, for adding a property to a collection. I might suggest Collection.add_select_option(prop, value, color=None) and Collection.has_select_option(prop, value) for checking existence.

Longer-term, I'd love to have an object-oriented way of dealing with a Schema, which would be a good place to handle this sort of thing, but great to cover this common use case in the meantime.

[jamalex]

This is now possible via #252 -- if you'd like to add anything on top of what is there, please feel free -- thanks!