Concrete

* Added section-level `LanguageIdentification` field
to allow language-specific analytics to run over meaningful
content.

* Added new fields to [email.thrift](thrift/email.thrift) to better
support/describe email messages.
* Added a new type, [SpanLink](thrift/structure.thriftL457), to
support linking between documents (e.g., mapping URLs within a
document).
* Added documentation about how to support pro-drop.
* Fixed a few documentation issues.

Added an optional field, [ConstituentRef](thrift/structure.thriftL91), to both
`MentionArgument`s and `SituationMention`s. This field allows references to
Constituent objects within a Parse object.

Added an optional field, mentionSetId, to EntitySet. This field stores
the UUID of an EntityMentionSet.

When present, this field indicates that all Entities in the EntitySet
contain mentions in the referenced EntityMentionSet.