v1.3.1: patch for aggregator issues

We found some issues with 1.3 (aggregators interacting with nulls/errors, primarily) that we didn't want to let sit until 1.4. So we fixed those issues and a couple others, and added a few 'minor' features. This patch version should be a definitive improvement over the base 1.3 version.

The complete list of changes is in the CHANGELOG. Here are the new options and features:

• new extend join type keeps the type of the first sheet, extending it with columns from the other sheets
• rename-sheet command (thanks to @jsvine for suggestion; what default keybinding should it have?)
• [DirSheet] add reload-rows (gz^R) to undo modifications on all selected rows
• remove all options.foo_is_null and add options.null_value
• options.save_errors (default True) to include errors when saving
• add options.json_indent for json pretty-printing