Alias support phase two: support for external commands

This issue serves as an epic-level tracker for adding "shell" aliases to gh; ie, aliases that can
invoke other commands and compose them.

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 (#936). 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 issue). This division mirrors git's approach to aliases.

• audit scriptability of individual commands Scriptability audit #940
• improve scriptability of individual commands (sub issues once audit is done)
• gh alias set <alias> "!sh -c ..." implementation shell version of gh alias set #941