Added feature to allow default extensions

[dploeger]

This PR adds a feature to allow a default extension for files, that have no extension set after filtered.

The need for this feature came along, because we wanted to host the generated documentation in another generated documentation and that didn't work without the .html extension for HTML files in links.

This PR adds an option called "defaultExt" which can be set to e.g. ".html" and will set this extension in the _href data for pages, that have no extension (e.g. Markdown-sources). Generated Index files are left out.

[dploeger]

I added a test for it. However, the test suite doesn't succeed, but I believe that's not due to my changes.

[dploeger]

@jldec Could this be merged?

[jldec]

Hi @dploeger thanks for the inquiry - sorry for the delays - this projects has had to take a back seat for a little while. Your PR suggestion looks reasonable - the only only change i might ask for the defaultExt option to live on the source intead of the root of the opts object.

I'm in the process of doing a general housekeeping upgrade of the modules in pub-server/pub-generator - hopefully that will allow me to fix the failing test and then merge your change.

[dploeger]

Thank you.

I've integrated the additional change you asked for.

[jldec]

Setting source.defaultExt to .htm or .html will create hrefs with one of those extensions, instead of no extension.

The same is also possible by setting an explicit extension for the href in the markdown file frontmatter.

See pub-server v2.11.3 or pub-generator v.4.3.5