Add Strokes, String Numbers and Fret Hand Finger Numbers

frethandfingers.js - new class to add fret hand fingers in notation
modifiercontext.js - update formatStrokes, add formatStringNumbers and
   formatFingerNumbers
renderer.js - add end line types and drawDashedLine function
stringnumber.js - new class to add string numbers in notation
flow.html - add tests for strokes, string and fret hand finger numbers
stringnumber_tests.js - add tests for stringnumber and frethandfinger
    classes
strokes.js - add tests for strokes class