Move ISLCObjectNode arrowOptionKeys to constants?

[jessegreenberg]

I believe the options defined in arrowOptionKeys are not instance specific and it might be more conventional to have these documented as a constant above the constructor. @mbarlow12 thoughts?

for #30

[jessegreenberg]

Ditto for pullerOptionKeys?

[mbarlow12]

@jessegreenberg I'm a little confused on the question. Almost all those options have different values depending on which object and/or sim is creating the instance. I suppose we could create some kind of enum type and simply pass a 'left' or 'right' option at instantiation that would handle the rest of them. Is that the idea?

[jessegreenberg]

My thought was that the actual array arrowOptionKeys does not change for each instance, those strings are just iterated with _.pick. It also might be easier to find the arrow specific options if they are closer to the top of the file.

ALIGNMENT_CONTAINER_OPTION_KEYS in AlignBox is kind of what I was thinking.

[mbarlow12]

Ah, yes. Sure thing!