Control handling of new lines

[johann-petrak]

It would be very useful to have a parameter which allows for setting a global or per-column strategy of how to handle newlines within the values to be shown in the table.

The strategies I would find most useful are:

• escape: instead of adding the newline as is, replace it with the string "\n"
• br: instead of addingthe newline as is, replace it with the string "
  "
• space: replace the newlines with a space

Currently this has to be done by a manual preprocessing step of the table data, but allowing for delegation to the tabulate library would be very useful.