Refactor RubyCommand#create_rb API

Using |options| instead of two parameters only adds code now, but it
will be more flexible in the future. We'll need this flexibility in the
next commit.

Refactoring suggested by Josef Reidinger.
Allow to convert modules with private symbols exported

Exporting private symbols from modules is usually needed because the
testsuite uses them.
Convert Sequencer.ycp with exported private symbols

Some tests access Sequencer::docheck, which is private.
Merge pull request #413 from yast/export-private

Fixes https://github.com/yast/y2r/issues/11
The y2r counterpart is https://github.com/yast/y2r/commit/ad0546afbdd054b4f4d7c934885d115f13de24f1