Support system level tool-version config file

**Is your feature request related to a problem? Please describe.**
When using asdf as a system-wide install (like #577), there isn't a simple way to set system tool versions.

**Describe the solution you'd like**
I'd like to be able to use a system level versions file, e.g. `/etc/tool-versions`.
So each user on my machine would have shared defaults.
Like how `git` supports local, global and system level config, with system config being stored in `/etc/gitconfig`.

**Describe workarounds you've considered**
Currently I can symlink a file to `~/.tool-versions` for each existing user.

The issue names ~/.tool-versions as the current per-user location and /etc/tool-versions as the desired system-level location. Start by tracing how asdf discovers and loads the versions file; done means system-wide defaults are read for users without overriding local configuration, with existing per-user behavior preserved.

Written by the indexing model from the issue text.