Datastore: Add interface for exporting custom formats

Currently the datastore can export data in multiple formats (#3390) but some interesting ones are still missing (e.g. Excel, LibreOffice). During a discussion on the ckan-dev mailing list, @wardi suggested to allow implementations of additional export formats via an interface.

Here's how such an interface could look:

• datastore_get_export_formats() would return a dict that maps format identifiers (e.g. xlsx) to dicts. The latter contain a UI-label for the format (e.g. {'label': 'Microsoft Excel XLSX'}).
• datastore_export(format, response, fields, options) would write the datastore fields to the response using the given format. options is a dict that contains additional options (e.g. the bom option from ckan/ckan#3390).

Start by reviewing the existing datastore export behavior and the proposed datastore_get_export_formats() and datastore_export(format, response, fields, options) entry points. Define how format identifiers, UI labels, response output, fields, and options—including bom—are represented, then verify that an additional format can be supplied through the interface.