Custom mime type for extensionless files

Hello,

I'm looking to do hosting on S3 without extensions on URLs. What would be really nice here is a custom mime type for files with no extension, that way I can have my html pages created without an extension and just served normally.

The solution I've got currently is this, in lib/middlewares/route.js:
`res.setHeader('Content-Type', extname ? mime.lookup(extname) : config.extensionless_mime_type || 'application/octet-stream');`

and this in _config.yml:
`extensionless_mime_type: 'text/html'`

Would this be a suitable feature request and implementation? If so I can submit a pull request.

Cheers,
Matt