New feature to tracking sample

Now initial bounding frame can be given in command-line, as an alternative
to manual selection via mouse. Bounding frame is given as optional last
argument in the format "x1,y1,x2,y2" where x's and y's are integers,
symbolizing bounding box with opposite vertices (x1,y1) and (x2,y2).
Merge pull request #18 from nailbiter/feature

New feature to tracking sample