Add support for field indexing via manual import

[zstumgoren]

This is a minimally invasive addition to the Dataset.import_data method and manual_import command. The change allows users to specify field-level indexes during bulk import, and opens the door to configuring field-level search from the web interface as well (if anyone is interested in that feature).

I had to add this support in order to sidestep performance issues during re-indexing a "large" dataset of about 18 million rows. I plan to file another issue related to re-indexing performance and/or inquire about the issue on the GGroup, but wanted to send this along in meantime in case you all wanted to integrate into core. Pull request related to this issue to follow shortly.