Support multiple styles applied to multiple selections

[sethaxen]

3dmol.js supports syntax like the following:
https://3Dmol.org/viewer.html?pdb=1YCR&select=chain:A&style=cartoon;stick:radius~0.1&surface=opacity:0.8;colorscheme:whiteCarbon&select=chain:B&style=cartoon;line&select=resi:19,23,26;chain:B&style=stick&labelres=backgroundOpacity:0.8;fontSize:14.
Here each selection selects some atoms, to which the subsequent styles are applied until the selection is reset, which allows sophisticated styling. It would be nice if the syntax here supported these kinds of complex styles and selections.

[jgreener64]

It's a good idea, I've added it to the todo list for this repo, I might get to look at it over Easter.