Added .setoptions() and .setoption() along with a few default options.

This addition allows for the labels of the buttons to be set by calling

introJs().setoptions({nextLabel: 'This Way', skipLabel: 'Go Away', prevLabel: 'That Way'}).start();

Or, individually like

introJs().setoption('nextLabel', 'This Way').start();
Merge branch 'label_customization' of https://github.com/casiotone/intro.js into casiotone-label_customization
Fix coding style + add `tooltipPosition` option
Add `default` to switch case statement to prevent wrong position in tooltips