Lets users configure the doc page link extension to achieve 'clean' URLs...

Lets users configure the doc page link extension to achieve cleaner URLs, e.g. '/api/v1/users/create' instead of '/api/v1/users/create.html'.

Without any configuration changes, Apipie will do exactly what it did before (so this shouldn't hurt anyone's deployment).

If `config.link_extension` is set to `""` in an Apipie initializer, API doc URLs will not have `.html` at the end.  

Regardless of this configuration setting, staticly-generated API pages (from `rake apipie:static`) will still use the `.html` extension. 

Addresses #216