Support input from stdin

[38tter]

Currently, input files are limited to be given from docs/mermaid directory, but parsing er diagram from stdin like:

$ bin/rails g mergration:install <<- EOF
  erDiagram {
    nantoka {
      int id
      string hoge
      string fuga
      datetime created_at
      datetime updated_at
    }
  }
EOF

seems more useful.