Add envs command

[hidakatsuya]

This PR add a new command envs. This command shows the list of defined environments in .extensions.rb.

plugin :hello_world_plugin, github: { repo: "hello/world_plugin" }

env :stable do
  plugin :hello_world_plugin, github: { repo: "hello/world_plugin" , branch: "stable" }
end

env :staging do
  plugin :hello_world_plugin, github: { repo: "hello/world_plugin" , branch: "staging" }  
end

If there is .extensions.rb with above definition, executing rex envs command will output the following:

$ rex envs
default
stable
staging