Add description for convert plugin
Add tests that image path is not 1st param.

If you write `&image(rounded,class=custom,URL,alt-text)`,
3rd param should be set for image path.
Modify how to check params in parseParams function.

Change foreach statement in parseParams() to check only type and custom class.
Then check params for setting path and alt text.
Add tests that lack pattern params order & too many params