v1.5: plugins, saving errors, and bugfixes

For the most part, this is a bugfix release. A few other improvements are here as well:

1. Basic plugin support

   • ~/.visidata added to sys.path on startup
   • copy plugin to e.g. ~/.visidata/vdfoo.py
   • add import vdfoo to .visidatarc to activate

2. Error saving

   • for tsv and json, errors are saved as options.safe_error string (default #ERR)
   • if that option is an empty string, error message is saved instead

3. Mouse improvements

   • click now moves to column also
   • right-click to edit cell, rename column, or rename sheet, depending on where clicked.

4. xls/xlsx loaders now use options.header to determine number of rows used to set column names (default is 1).

See the CHANGELOG for a more complete list of bugfixes and changes.