Allow updateSlider to go all the way left or right

[scotself]

Line #338 in juxtapose.js, in the updateSlider function, is there a particular reason it's
if (leftPercentNum > 0 && leftPercentNum < 100) {
instead of
if (leftPercentNum >= 0 && leftPercentNum <= 100) {
?

The way it is now, I cannot make a button which, on click, slides the slider all the way right or left ('show only before' or 'show only after' button). For now, I just grabbed the code (great work, btw!!) and made a copy of our own and made this change locally, but that would be a simple and welcome change. Thanks!