Add option to export to CSV via sleap-export and API

[talmo]

In #1438, we added a feature to export tracks to CSV format from the GUI.

It would be great to be able to do that via the sleap-export CLI and the Labels.export() API.

For sleap-export, we just need to add CSV as a target format and pass in csv=True in the export call here.

For Labels.export(), we probably want to extend what's going on here.

We should also be sure to support the new functionality being added in #1624.

Relevant: #1627

[eberrigan]

#1730: Issue related pull request.