Add support for extensionless HTML files

Add support for an optional `trim_file_extensions` setting to cause plugin to generate extensionless HTML files that can be served directly without multiviews, .htaccess rewrites, etc. To enable, add the following line to _config.yaml:

    trim_file_extensions: true

When this flag is absent (or false), the plugin's existing behavior is unchanged. When `trim_file_extensions` is true:

1. Tweak plugin behavior to generate extension-less HTML files.
2. Mod WEBrick server to return a "text/html" content type for these files since it can no longer be inferred from the extension (Under Apache users would need a `DefaultType text/html` or similar server apparatus, this handles the `jekyll serve` case.)