Allow keeping output seperator

[ChrisJefferson]

I want to use 'tabulate' to form table for latex papers (which use a seperator &), so it is easier for me to read them from the source.

However, for this, I need the seperator leaving in. The linux-util column program has the option:

-o, --output-separator string
          Specify the columns delimiter for table output (default is two spaces).

Which I can use to set a separator (which is inserted after whitespace has been used to nicely line up the column of course)