Fix options with vector and optional<vector> targets

[mmahnic]

Fix problems reported in issue #17.

Fixed

• The optional targets are now filled correctly.

Changed

• For options with vector targets the default count changed from minagrs(0) to minargs(1). For
  options with optional<vector> targets the default is still minargs(0).
• When an option with a vector target has minargs(0) a flagValue is added to the vector only if
  the vector is empty.