Add ExposeMissing context manager

[lafrech]

• Use Python 3.7's context variable feature to create a context manager in which a document returns missing for missing data rather than None
• Use this context manager in schema_from_umongo_get_attribute

This makes the Document responsible for outputing missing rather than the schema having to guess. Also the schema works with any type of data, not only a Document instance (with other data, the context manger is no-op).

I hoped to remove the whole mongo_world management thanks to this, but there are other issues with data from Mongo vs. Document.

The benefit is not as great as I expected, but I still like this implementation.

[coveralls]

Coverage decreased (-20.9%) to 74.852% when pulling 021a3b3 on expose_missing into 8a0aaf9 on master.

[coveralls]

Coverage increased (+0.06%) to 95.611% when pulling 07f2715 on expose_missing into 53a8c4e on master.

[lafrech]

This implementation is totally sub-optimal. It misses a nice point of the context variable, which is that we could put the context manager at the dump method level rather than the get_attribute level. This is another reason to like the context_var approach better than the current one.

[lafrech]

Rebased on master.

• Now uses the context manager in schema dump method.
• Removes now useless schema_from_umongo_get_attribute.
• Exposes (SchemaFomUmongo renamed as) RemoveMissingSchema.
• Makes RemoveMissingSchema an opt-in. Easy to achieve by passing it to custom BaseDocument and BaseEmbeddedDocument.

[lafrech]

Finally, this is the first part of a rework followed in #299 that removes mongo_world and nicely simplifies the code.