Linkie

- Update to only check links prefixed by one of `="`, `(`, `<`, ` `(a space).
- Linkie now finds all unique links at once, then uses multithreading to check them all.
- Linkie now rechecks links that had a `ConnectionError`, as these are often valid.
- Broken links in the SUMMARY are now also displayed with their status code.
- Dependency updates:
- Update `PyYaml` from 4.2b4 to 5.1.1.
- Update `requests` from 2.19.1 to 2.22.0.

- Update logging configuration.
- Update dependencies.

- Set User-Agent to emulate browser viewing.
- Use Python logging module.

- Allow passing of variable of config settings in Python.

- Update method for detecting URLs with brackets.

- Allow adding URLs to skip to configuration file.
- Skip checking URLs that have already been checked.
- Show connection error names instead of 999 status.
- Uses class based object allowing user to retrieve values after running.