break up script.js into more reasonable files

[sam-lb]

I don't know why I allowed it to get to this point (script.js is 1,846 lines as of right now and has several semantically different tasks)

One possibility:

• Leave everything that interacts with the DOM in script.js, take everything else out (move all DOM interactions out of the Plot class)
• Plot should get its own module
• All objects for use in p5 mode should get their own module (basically anything that inherits from Plottable
• validateInput, validateSliderInput, populateValueScope could get their own module
• data loading (shaders, sample images, json) could get its own module

script.js is still going to be a huge file after these changes, but it will be more organized.