Add additional rating systems as options

Problem/Motivation

The Text-Statistics Javascript that is being used for this project supports additional rating systems. These should be selectable via a global setting. Examples:
- Flesch Kincaid Reading Ease
- Flesch Kincaid Grade Level
- Gunning Fog Score
- Coleman Liau Index
- SMOG Index
- Automated Reability Index
- Dale-Chall Readability Score
- Spache Readability Score

User interface changes

- UI in global settings to change which rating system to use
- UI to change the rating system in the rating area but also the link to the explanation of the system in the rating area.

API changes

None

Data model changes

- Config option changes
- Structured data (JSON?) for ease of picking which system to use along with a explanation link.