Allow a MIME type to be specified for extensionless files

I'm using s3_website (thanks!) for deploying sites with 'clean' URLs, e.g. `www.domain.com/info`. Of course, uploading an HTML file without an extension results in a blank Content-Type header. This tweak allows a config option to specify the default MIME type for files with no extension. From the revised README:

### Specifying a MIME type for files without extensions

`s3_website` will look up the MIME type of each file it uploads, and infer the Content-Type from it automatically. By default, files without an extension will have a blank Content-Type.

You can specify a default MIME type for files without an extension using a line like this in `s3_website.yml`:

```yaml
extensionless_mime_type: text/html
```

This is useful when you are uploading HTML files for which you want 'clean' URLs, e.g. `www.domain.com/info`.