Use ARGF instead of cheking ARGV and STDIN

Do you know [`ARGF`][1] of Ruby? How  about to use it instead of checking `ARGV` and `STDIN` by yourself for readability?

But this patch introduces backward incompatibility:

    $ git checkout master
    $ bundle exec nora2html example/*.nora | wc -l
    17
    $ git checkout argf
    $ bundle exec nora2html example/*.nora | wc -l
    628

because `ARGF.read` returns text concatenated from multiple files.

If it is okay, could you consider merge this patch?

[1]: http://docs.ruby-lang.org/ja/2.1.0/class/ARGF.html