Need to set a safe line separator for tabular entity/contact handler

[eblondel]

For the time-being ';\n' is used, but not enough generic as ';' or '\n' are likely to be used as content.
A default line separator has to be set, and should be modifiable as option for user preferences.

[eblondel]

@wheintz @juldebar @juliettefabre i'm going to set this default line separator as _\n . If you use another line separator, you can set it very easily as option in your config (see https://github.com/eblondel/geoflow/blob/master/inst/extdata/config_example_gsheets_action_geometa-geoserver-geonetwork-zenodo.json#L11)