Add ability of templates to set the extension of resulting files

[mercmobily]

At the moment, the result file name extension is always .html. However, we already have a case where we have the plain template that outputs to .md. So, the result is formally incorrect.

Ideally, templates should know what kind of files they create. However, the "ideal" solution would imply:

• Some kind of config file for templates
• Ability to specify the resulting extension

The trouble is that the only possible extensions in practical terms will only ever be .md and .html. Plus, .md is a "strange" case. So, we are potentially building a whole config file that will never ever grow to more than one option, and it's only really there to deal with a special case.

The only other solution is to allow layouts to be named like plain.md where the extension will be the resulting file extension. But, I don't like the idea of having directories with extensions.

Another solution is to force .md to any layout where their name starts with plain. Still, not ideal.

The current solution is to use -x to set the extension.