[now-cli] Add now env command

[styfle]

This PR adds a new command now env for managing environment variables for a given project. This will complement the changes in the Dashboard and will provide 3 possible environments: production, preview, and development.

• Add now env ls
• Add now env add
• Add now env rm
• Add now env pull
• Add test for now env ls
• Add test for now env add
• Add test for now env rm -y
• Add test for now env pull -y
• Add test for now env add FOO preview < secret.txt
• Add test deployment to verify env var is assigned to both build and runtime
• Test on Windows

  ▲ now env [options] <command>

  Commands:

    ls      [environment]              List all variables for the specified environment
    add     [name] [environment]       Add an environment variable (see examples below)
    rm      [name] [environment]       Remove an environment variable (see examples below)
    pull    [filename]                 Read development environment from the cloud and write to a file [.env]

  Options:

    -h, --help                     Output usage information
    -A FILE, --local-config=FILE   Path to the local `now.json` file
    -Q DIR, --global-config=DIR    Path to the global `.now` directory
    -d, --debug                    Debug mode [off]
    -t TOKEN, --token=TOKEN        Login token

  Examples:

  – Add a new variable to multiple environments

      $ now env add <name>
      $ now env add API_TOKEN

  – Add a new variable for a specific environment

      $ now env add <name> <production | preview | development>
      $ now env add DB_CONNECTION production

  – Add a new environment variable from stdin

      $ cat <file> | now env add <name> <production | preview | development>
      $ cat ~/.npmrc | now env add NPM_RC preview
      $ now env add DB_PASS production < secret.txt

  – Remove a variable from multiple environments

      $ now env rm <name>
      $ now env rm API_TOKEN

  – Remove a variable from a specific environment

      $ now env rm <name> <production | preview | development>
      $ now env rm NPM_RC preview

[codecov]

Codecov Report

Merging #3999 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3999   +/-   ##
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  Coverage   13.13%   13.13%           
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  Files         290      290           
  Lines       10312    10312           
  Branches     1348     1348           
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  Hits         1354     1354           
  Misses       8865     8865           
  Partials       93       93           

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[lucleray]

👌