Fix downloading functionality, add DNS note in README

[binimum]

Fixes

Fixes issue #9.

Description

1. Z-Lib and LibGen have updated their UIs so code has been updated to keep these on track.
2. Remove libgen.li downloading due to Cloudflare 'Phishing Warnings'.
3. On Debian (RPi armhf), calling WebStoreDialog returns an error. Not tested on other platforms. Add a try except clause to catch this error and open it in an external browser instead.
4. Remove url extension checking in favour of MIME checking because many links don't actually have an extension.
5. Update plugin config accordingly.
6. Add a note in the README about DNS.

[amiantos]

For what it's worth, currently (for me) Z-Library downloading doesn't work and libgen.li downloading is the only option that works.

[markferry]

Thanks for this.

Did you notice you inadvertently switched all the line endings?

[binimum]

I didn’t, I authored this commit on the Pi I was using to run Calibre 😅

Also I haven’t maintained this patch for a while now, I think Z-Lib doesn’t work anymore.

[nickbwuzhere]

Libgen.rs / Libgen.se links do not work for me, we could fix this to test the links for connectivity and if they don't work, add them to an ignore list. Libgen.li is the only one that works for me, so not a good idea to just skip over it as this would break downloads completely for some people. We could also easily extrapolate it out to a setting in the plugin, where we can select the anna's archive external sources that work fine for us, and deselect the ones that don't, then change the if/else logic here to just use the ones selected in the settings.