Added onOptionSelected event to DialogueRunner

Please check if the pull request fulfills these requirements
[ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Does it pass all existing unit tests without modification?
- If not, what did you change?
- If you altered it significantly, what coverage issue did you fix?
[x] Docs have been added / updated (for bug fixes / features)
[x] CHANGELOG.md has been updated to describe this change

What kind of change does this pull request introduce?
[ ] Bug Fix
[x] Feature
[ ] Something else
What is the current behavior?

No event is raised when an option is selected by a Dialogue View.

What is the new behavior (if this is a feature change)?

An new event is raised by the Dialogue Runner when an option is selected.

Other information:

Adding the onOptionSelected event allows multiple components to react to a selected option without coupling them to the Dialogue View responsible for the selection. Specifically, this allows multiple Dialogue Views to handle the RunOptions method without being coupled to one another-- they can be notified if another Dialogue View selects an option and take action accordingly.

Important notes:
- The event is raised before the call to Dialogue.SetSelectedOption, which may need some consideration. The thought was that subscribers would want to handle their own cases before the state of the Yarn program was updated in any way.
- The event only sends the index of the option. If a subscriber needs more information about the option itself, it must obtain it some other way (such as implementing RunOptions, if the subscriber is a Dialogue View).
Link to documentation pull request: https://github.com/YarnSpinnerTool/YSDocs/pull/58

YarnSpinnerTool / YarnSpinner-Unity