CSV custom delimiter support

CSV and TSV are very similar formats. Some people need tab separated values, some (me) need `;`-separated values.
So I've come up with the idea of having one interface for all kinds of \*SV formats (delimited by any character you need).
The idea is to keep csv and tsv format in the Dataset class and also add functions `set_*` and `get_*` (e.g. `set_csv`, `get_csv`) which allow to use custom arguments (you can't do it with a property). In my case I've added `delimiter` argument.

There are also two testcases showing how to use it. Other tests also pass, so hopefully I haven't broken anything :-).

Please tell me what you think. Is this idea OK? As I said just need `;`-separated values support, but maybe someone else will need `:` or `^` or anything-else-separated values support in the future. Perhaps there is a better way to provide it, but I haven't figured out anything better.