Add Cross-Platform/Distribution Compatibility

[jblakeman]

Currently, apt-select must be run on an Ubuntu operating system. However, we could just check the platform and only allow for file generation if run on an Ubuntu OS while still offering full reporting functionality when run on other platforms or distributions.

[jblakeman]

Without having the Ubuntu release name set, pulling correct information about mirrors from Launchpad would either need a default release name or have a target release command line option. Since alternative mirrors are usually inconsistent at the frequency at which they update each release, using a default release name could lead to misleading status reports.

Alternatively, non-Ubuntu distributions could default to latency only reporting.

A better solution might be to use latency only reporting for non-Ubuntu systems unless a valid Ubuntu release name is specified as a command line argument.

[luckydonald]

Could the platform be set as flag?
I.e. run on the server uname -v
And use

apt-select --kernel="#1 SMP Debian 3.16.43-2+deb8u5 (2017-09-19)"

Is there some way to run it outside of linux, to just get a list of mirrors?
Basically a --ignore-not-linux?
Trying to run it on a mac next to the server. Can't install python on the server.