Add support for .single returing an Option[R]

[samikrc]

Currently the .single method return a single row only, and errors out if there is 0 or 2+ rows. Request to change the response to an Option[R] signature, where for 0 rows the return will be None, 1 row, return the row, and for 2+ rows, return an assertion error.

The proposal is to introduce a runOption method in DbApi, which will provide a .singleOption method. This method witll return an Option[R].

[samikrc]

Raised an initial PR: #34
@lihaoyi Please comment if the approach is correct. Will add test case once the approach is finalized.