Support revealjs min/max scale, weight, height & margin

[cmoulliard]

margin: '#{attr 'revealjs_margin', 0.2}',
width: '#{attr 'revealjs_width', 960}',
height: '#{attr 'revealjs_height', 700}',
minScale: '#{attr 'revealjs_minscale', .2}',
maxScale: '#{attr 'revealjs_maxscale', 1}',