Split tldr/linux.txt into distribution-specific files

[khughitt]

The current linux.txt includes commands that are geared towards Debian/Ubuntu installations (e.g. apt-get), but are not useful in other distros such as Arch, Redhat, etc.

Rather than assume one particular flavor of Linux, instead it might be useful to try and detect which flavor of Linux the user is using, and then load a distro-specific template, if one exists. If not, then fall back on a distro-agnostic file which contains only commands that do not depend on the specific distro the user is using (so no package manager commands, for example).

To avoid redundancy, you might consider adding a second layer of "common" commands -- this way you could load a linux_common.txt + linux_<distro>.txt (if the distro is known and supported).

Interested users of alternative distros could then submit PR's with distro-specific commands.