Allow specifying different mirrors.txt

[Zitrax]

For me the hardcoded URL currently only lists 5 mirrors. However I see there are many different lists, for example http://mirrors.ubuntu.com/VA.txt with 183 mirrors which found several faster ones than the default. Now I don't know exactly what that VA list is - but anyway could be nice to be able to pass an arbitrary URL with a list on the command line.

[jblakeman]

I think VA is the 2 letter ISO country code for Vatican City. At mirrors.ubuntu.com there is a list of mirrors for each country, labeled by country code.

Being able to pass in any URL would require extra validation steps, but I think adding a country code argument that specifies which mirror list to use could be a good solution.

[Zitrax]

That might be fine since VA currently contain so many of the mirrors. I was looking for something that checked more mirrors than the very limited default (at least for my region). But if they change VA to only include a single Vatican mirror the problem is back, since I would then have to call the script once at least for each neighboring country including my own and compare the output.

[jblakeman]

Maybe, it could be passed a list of country codes like --country AT,CH,DE,IT,FR which would test all the unique mirrors found in that collection of lists.

[Zitrax]

That sounds good to me.