Basic Alias Support

This issue serves as an epic-level tracker for adding basic alias support to gh.

We're dividing alias work up into two major, sequential buckets. The first is support for "internal"
aliases; ie, aliases that expand to invocations of gh itself (this issue). The second bucket is support for
"shell aliases," where users can opt into shell expansion such that gh can be combined with other
commands. This division mirrors git's approach to aliases.

This particular issue covers the first bucket. In addition to basic support for aliases, we're
including in this first bucket commands that make it possible to share aliases between users.

• Split auth config into its own file. We want to enable people to store their gh config in a
  dotfile repo but that's precluded if we store an auth token there. Split config file - to separate credentials #937
• gh alias set <alias name> <expansion>. gh alias set #938
• gh alias list - print the yaml of the aliases section of the config gh alias list #989
• gh alias delete <alias name> gh alias delete #990

Follow-up epic: #939