Ability to use stdin instead of file

To also use config, I could specify `-f` parameter, but if there is stdin, use that instead of file with settings from config. Something in this example I tell it to use settings as `for src/Main.elm`:

`cat somefile.elm | elm-static-html -f src/Main.elm > file.html`

And also using output like this will be possible after #8 is fixed.