Smarter Superscript and Subscript Alignment

[xavierholt]

Isotopes and charges (and presumably any other super- or subscripts) push the atoms they're applied to out of alignment. This tends to work well when text is drawn from left to right or vice versa, but not when the atom symbols are stacked vertically. For example:

[13CH2]([CH2-2][13C@@H]([13C](=O)O)N)[13CH2]N

I propose we should ignore super- and subscripts when centering vertical text. This will make the atom symbols line up, which should be much nicer aesthetically. There'll still be a potential for charge supersciripts and count subscripts to clash, though (as in the [CH2-2] above).

[xavierholt]

Any fix to this should probably include a fix for #195, and vice versa.