Allow to pass multiple input files via globbing

Following some discussion on issue #14, I would like to change the API to allow to pass multiple input files via shell globbing. This will be a breaking change because currently the second input argument is treated as the pdf output filename.

$ md-to-pdf **/*.md

The shell would expand the glob into an array of files that will be passed as input arguments (leaving the glob expansion to the shell rather than dealing with it using some npm package).

[simonhaenisch]

Ah, I see you had already found this issue! Thanks for dropping the info here (:

[karlhorky]

To do this with the current version, you can do this with a shell for loop in your npm script:

{
  "scripts: {
    "build": "for file in dir/*.md; do md-to-pdf $file; done"
  }
}

Ref: https://github.com/karlhorky/npm-scripts-tricks/blob/master/readme.md#run-a-command-for-all-files-in-a-directory