Make entry_title_template load from a file in the src/scriv/templates directory

[DonalChilde]

It would be handy to have the entry title template load from a file like the new_fragment templates do. This would provide an easily found example of the template in the src/scriv/templates directory, making it easier for new users to find and customize the template. Right now, you have to search the source code for the template string. That is not exactly hard, but took me a few minutes :) .

Alternately, a doc example of a custom title template file, with the necessary .toml entry, would serve the same purpose. It became obvious after finding the code, but I did not find the possible values passed to the template (version and date) listed in the docs. This could be a RTFM error on my part.

[DonalChilde]

This might also give more control over whether to output the <a></a> as part of the title. I personally do not use this, and have to delete it each time.