Added various [Spiget] badges (#2745)

* Spiget Downloads badge

* Spiget Download Size badge

* Spiget Latest version badge

* Add a base class

* Spiget Rating / Stars badges

* Spiget versions badge

* remove useless regex escape

* use renderVersionBadge for rendering

* misc fixes

* use expectJSON when possible

* use the download count color formatter

* merge service classes

* add keywords

* Add tests for non-star ratings

* Add nock tests

* misc fixes

* chore: minor formatting update on spiget dl