add env command, to output environment to stdout for use by shell

[djerius]

At times it's useful to set up a shell with the carton local environment.

carton exec bash doesn't always work because it requires that the bash startup files pay attention to the existing environment. This commit adds an env command which outputs shell commands to set the PERL and PERL5LIB environment variables. It's used thusly:

eval $(carton env)