Added image size

Makes image size possible. The arguments look like this:

imageSize: {
    width: 500,
    height: 500
}

If nothing is specified, that dimension is considered infinite. You can also explicitly specify the string 'infinite' to make a dimension infinite. It is possible to have just one dimension infinite.

I haven't tested exporting but it should work. I'll verify that when I fix the merge conflict tomorrow, but I want to get this out for review before I go to bed.