Remove leading and trailing hyphens from slugs to match jekyll behavior

[sirwart]

When I tried to use gojekyll with my blog, a post that had a question mark at the end had the wrong slug and therefore links` to the post broke. This was due to the fact that jekyll will strip leading and trailing hyphens after it replaces non-alphanumeric with hyphens. You can see this behavior here:

https://github.com/jekyll/jekyll/blob/646b4241669ac54247d8bf1ce3e4c616ac71d9fe/lib/jekyll/utils.rb#L219

To fix this, I added similar behavior to the Slugify function in gojekyll, and after that my site built with the correct routes.

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes.
• New and changed code is covered by tests.
• Performance improvements include benchmarks.
• Changes match the documented (not just the implemented) behavior of Jekyll.

[danog]

Thanks!