Adding an optional playbutton param, to show a play button

Having the song play after page load was a little annoying for me when I used the embeddable option.  This commit introduces a new `&playbutton` param.  

If present a link labelled **▸ Click to play** is appended and the tune will not start until you click it.

A new global object `songOfGitHub` was introduced to deal with this, and `allWeeks` array was pulled up.