Add syntax for abstract line labels

[cdibbs]

Right now, you can include line labels or not. They're non-semantic but permitted for readability:

1 a and b: premise
2 | ~b: assumption
3 | b: and elim2 1
4 ...

What we'd like is the ability to label only those lines we need to reference. A syntax like this:

@1 a and b: premise
  |  ~b: assumption
  | b : and elim2 @1

where @1 could also be, e.g., @firstprem or @prem1. We might also allow them on preceding lines, to further increase readability:

@firstprem
a and b: premise