Exec instead of formatting environment variables to eval

[gabrielg]

This modifies aws-env to set the environment variables and exec the given command and arguments, rather than forming a string to pass to the shell's eval. This saves us the trouble of having to escape the string for the shell.

This PR pretty significantly changes how aws-env works, so entirely understood if you'd rather not merge it.

[orfin]

Thanks @gabrielg :-) When working on aws-env we also considered such approach but we decided it will be less risky to not rely on Go program (e.g. memory leak on aws-env could cause problems for long running processes). But you are right - on the other hand, eval introduces trickier work with escaping ENV parameters etc.

It may be something that we could add as another branch? WDYT?

[orfin]

@gabrielg WDYT about adding additional 'exec' options flag?

Something like aws-env --exec|-e

AWS_ENV_PATH=/prod/my-app/ AWS_REGION=us-west-2 ./aws-env --exec node -e "console.log(process.env)"

that won't break backward compatibility and will allow for that superb nice usage :-)