Added new class -show

Added a new class (scrollyou-show) that sets the default transparency for the scroll bar to 0.5 all of the time.
Added option 'show'

Added additional option 'show' that adds the 'scrollyou-show' class to the scroll bar to make it visible all of the time, not just when hovered on.
Added option description

Added an entry for the new 'show' option and a description.
Update README.md
Merge pull request #2 from TheBrockEllis/master

New option 'show'