fetchart: Choose / convert image formats

At present, the ```fetchart``` plugin is oblivious to the format of the images it downloads. While JPG is most common, one may come across cover art in other formats too. For example, MusicBrainz [carries](https://musicbrainz.org/doc/How_to_Add_Cover_Art) GIF, JPG, JPEG, PNG, HTM, HTML, JPE, JFIF and PDF.

Some players, notably in-car entertainment systems, may be picky about the format they display. For example, BMW supports JPG but not PNG. Hence the need to convert the downloaded cover art.

It would be useful if the ```fetchart``` configuration had two extra options:

- **format**: Download images of this format only (gif, jpg, jpeg, png, htm, html, jpe, jfif, pdf)
- **convert_to**: Convert downloaded image to this format (gif, jpg, jpeg, png, htm, html, jpe, jfif, pdf)