Add full support for the `@example` tag

I have built on the awesome work done by @thebeline and @siad007 in PR #1227 and PR #1145 and added the following:

- Now uses the new configuration objects
- Now supports setting multiple example folders instead of one
- Example folders can be set in the 'files' section of the configuration
- All essential classes are now unit tested
- Moved determination of example location to a new Finder class
- Rewired the classes to make better use of DI
- Generated output is now in Markdown and not in HTML to increase portability

> As per @siad007's and @thebeline's original pull request, this fixes: #48 and #369, closes #1145, and supersedes #1209 and #1227.