bake environment stacks

[rsgalloway]

add an option to bake one or more environment stacks down to a single .env stack file:

$ envstack [STACK [STACK ...]] --bake baked.env

this could also be used to init a new stack file from scratch that could be updated:

$ envstack --bake test.env
$ cat test.env
all: &default
  HELLO: world

darwin:
  <<: *default

linux:
  <<: *default

windows:
  <<: *default