Allow plugin name alias

I hit an issue similar to https://github.com/buildkite/agent/issues/520 and solved it with the comment about using `ssh://`. While this totally works it gets pretty verbose as I use my plugin several times throughout the pipeline yaml.

Does anyone have thoughts on supporting a plugin alias with supporting config or env vars? Something like:
```
steps:
- command: "echo whee"
plugins:
alias://awesome_plugin:
...
```
With a plugin alias list in the agent config file.

I feel like this is sort of a nit-pick issue but I'm happy to drop in a PR for it. I mostly wanted to take a quick pulse check on the style/appetite for a change like this before I wrote any code.