Further improve handling of ports

- Distinguish register output ports in command-line reporting. This is
  a special case where a register (marked REG_SRC or REG_DST) is also a
  port.
- Limit port start and finish points to be the top-most instances in the
  hierarchy (eg i_data rather than <module>.i_data). This effectively
  removes duplicate port variables and reduces the number of fanin/out
  paths reported, which makes sense from a user point of view.

Add note on the extended test set

Merge pull request #4 from jameshanlon/path-restriction

Add options to relax path start and end constraints

Bump C++ version to 17