Compiler should support `-` as a file name to mean standard input

[KamilaBorowska]

If I run earl compile -np -, in my opinion it should read from STDIN and wait for CTRL+D to be input, rather than open a file called -. This is a standard UNIX convention.

However, instead it outputs a Node.js's error message.

Error: ENOENT: no such file or directory, stat '-'
    at Error (native)
    at Object.fs.statSync (fs.js:892:18)
    at compile_path (/home/olivier/npm/earl-grey/src/run.eg:229:12)
    at _run (/home/olivier/npm/earl-grey/src/run.eg:429:13)
    at Object.run (/home/olivier/npm/earl-grey/src/run.eg:312:4)
    at Object.<anonymous> (/home/xfix/node_modules/earlgrey/bin/earl:5:3)
    at Module._compile (module.js:425:26)
    at Object.Module._extensions..js (module.js:432:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:313:12)