Fix local config to not require .html in URLs

[seanredmond]

When running locally, you have to add .html to all URLs except for the index. For example the equivalent of http://tindallgrams.net/66-FM1-59 on localhost has to be http://localhost:4000/66-FM1-59.html Provide a configuration that allows you to run Jekyll on localhost without adding the extension.

Obviously without breaking the Heroku configuration.

[wolfgang42]

It looks like this has been fixed at some point—I can run bundle exec jekyll serve and all of the URLs seem to work just fine without .html on them.