Create command line app

[mgd020]

So user can run mjml and it does what the JS version would.

Options:
  -r, --read                 Compile MJML File(s)                        [array]
  -m, --migrate              Migrate MJML3 File(s) (deprecated)          [array]
  -v, --validate             Run validator on File(s)                    [array]
  -w, --watch                Watch and compile MJML File(s) when modified[array]
  -i, --stdin                Compiles MJML from input stream
  -s, --stdout               Output HTML to stdout
  -o, --output               Filename/Directory to output compiled files[string]
  -c, --config               Option to pass to mjml-core
  -V, --version              Show version number                       [boolean]
      --noStdoutFileComment  Add no file comment to stdout             [boolean]
      --help                 Show help                                 [boolean]