provider: add "url" provider

This patch add support for a new provider type "url".
It is now possible to fetch cores from url.
For example:

[provider]
name = url
url = http://nicecores.net/or2400.v
filetype = simple
corename = or2400

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
merge