Add hidden stdin option

[yhatt]

Add --stdin boolean option for enable or disable reading Markdown from stdin (true by default). By setting --stdin=false or --no-stdin, Marp CLI won't read content from stdin. It allows to avoid hang up while reading stdin in several environments.

--stdin is a hidden option because reading stdin is an expected behavior for most users using CLI. User can see help with --show-hidden if necessary. (It's a built-in option of yargs)

$ marp --help --show-hidden
...
Basic Options:
  ...
  --stdin            Read Markdown from stdin     [boolean] [default: true]

--no-stdin helps to resolve the problem for Now deploy environment and Marp for VS Code. Close #93.

ToDo

• Add tests