Manifest.mf - Add a section for specifying command env variables

[zeralight]

Currently the way to pass env variables:

cmds:
  - name: pretty-grep
    type: executable
    executable: bash
    args:
      - -c
      - export GREP_OPTIONS="--color=auto -r --exclude-dir=node_modules --exclude-dir=.git" grep

But we can have a cleaner syntax if we can specify environment variables in a dedicated attribute:

cmds:
  - name: pretty-grep
     type: executable
     executable: grep
     env:
       - GREP_OPTIONS: --color=auto -r --exclude-dir=node_modules --exclude-dir=.git