Add '--format' option to 'pstore import'

Add a `--format` option to `pstore import` as an alternative to the `--delimiter` and `--quote` options. It makes setting the input format to TSV easier than by changing the delimiter.

Besides TSV, the `--format` option supports CSV. Unlike the default input format, this input format support quoting. In other words, it is equivalent to setting `--quote` to `"`. (Quoting is disabled by default because it incurs a performance penalty.)