Add options for controlling svelte dominators output

[fitzgen]

• maximum depth in dominator tree
• maximum number of rows to include in output, regardless of depth

Each of these involves new fields on the Dominator struct in ./opt/opt.rs.

The DominatorTree struct in analyze/analyze.rs should make and hold a copy of the opt::Dominators options.

Then, the impl traits::Emit for DominatorTree should consult the new options and modify its output appropriately.

[fitzgen]

@sendilkumarn, this could be a good issue for you to pick up! :)