Add description support for record type messages

[ixzzd]

We at @ahrefs use have use messages descriptions quite often, and lack of description support was the blocker to migrate from object message type to record.

In this PR I propose to pass message descriptions to records with [@intl.description ...] annotation, ex:

let msg = [@intl.description "Description for message"] {id: "i.am.id", defaultMessage: "i.am.defaultMessage"};

In this case, only the extractor aware of message description, and we don't have to deal with {... description: None}, {... description: Some("description")} in the records, as we discussed before
cc @cknitt

[cknitt]

Thanks a lot for the PR! 😀 I like this idea! Could you also update the documentation in README.md accordingly?

/cc @alexfedoseev (FYI)

[ixzzd]

@cknitt I added an example to the documentation

[cknitt]

Thanks again for your contribution! 👍

[alex35mil]

Looking great!