Feature use rc config

Use [`rc`](https://github.com/dominictarr/rc) for configuration. Easily set config options is [multiple places](https://github.com/dominictarr/rc#standards).

For example

    $ node src/gistfy.js --port=3141 // Run the server on port 3141

Or

    In file ".gistfyrc"

    ; Serve html by default
    type = html