Downloads: add support to switch between proxies

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The package now has support for an optional setting where all requests are routed through a proxy. As described in this comment it is not currently possible to switch between proxies or trigger the proxy on demand with each request.

Trafilatura uses two different librairies under the hood: urllib3 (standard) and pycurl (optional). The latter can be easily adapted, however the former uses a connection pool for performance reasons which makes it slightly more difficult to add this new functionality.

Maybe somebody is interested in drafting a PR.
CC @andremacola in case you find some time.