xpath-based aggregator

[jjst]

Can be used for e.g. https://www.cheriefm.fr/webradios/mur-des-radios/

[jjst]

Cherie FM has https://www.cheriefm.fr/onair it turns out, so no need for above, see #15.

That said we'll need it eventually... along with full page rendering. I had a (WIP) patch with requests-html and pyppeteer, the day will come when it'll be needed.

[jjst]

Finally found 1 station that uses xml to fetch now playing info, which would require xpath support: https://www.jazzradio.fr/radio/webradio