Settings and Validation Update

• Added settings to configure the extension: full text or not, HTML or not, and entries per feed. (#9, #10)
• Updated URLs to be cleaner. (#3)
• Added Last-Modified header. (#7)
• Improved compatibility with futures versions of Flarum: rewrote code using deprecated APIs.

Thanks to @zerosonesfun for its contributions, and all other for their suggestions through issues!