Consider how stack plugins should work with global options and docker re-exec

[mgsloan]

Currently, stack plugins need to be the first argument, like stack run. If you use stack --stack-yaml ... run, it won't work - see this FIXME.

Ideally, we'd parse global options before invoking the plugin, so that:

• We can re-exec in docker
• We can use a PATH variable that includes the project's bin dir and the snapshot's bin dir. This way stack plugins can be installed to these locations.

We might also consider doing something similar to stack's current behavior with docker, where it will copy the stack binary into the docker container. Should it also copy the plugin binary?