Tab and other delimiters for frame.to_csv()

[justinsutw]

Currently, only comma delimiter is allowed.

Some of the scripts I am using are quite inflexible on the input file format and can't take datatable output files as is.

[oleksiyskononenko]

I guess we can at least allow any separators that are supported by dt.fread(): https://datatable.readthedocs.io/en/latest/api/dt/fread.html#sep