Remove line tracking

**Not ready for acceptance**

In issue #11 I proposed removing instances where line numbers were being tracked and passed around throughout the code.  This PR incorporates that proposed change.

Remaining thoughts/tasks:

- [ ] Decide whether this is actually a good idea
- [ ] Confirm that [this change](https://github.com/thephpleague/markdown/commit/cf00cd1b79d48aa110815e59c9bc5add01ca0a62#diff-64d43afca3fac22bce1c4d9a3b84310aR492) is valid and works as expected
- [ ] Test against the Parsedown suite (see PR #9), possibly adding some new checks