Adds start option & jump method.

This commit adds a "start" option that will automatically jump to
the desired position if provided when stackslider is initialized.
This option also depends on the newly added jump() method.  This
method accepts an index (int - zero-based) and will jump (uses
flowing) to the given item.
Merge pull request #4 from ericthelast/feature/add-start-option

Adds start option & jump method.