Remove mandatory extension "html" for template file

[usr345]

We are using your library for creating LaTeX files and are planning to generate markdown files. Is it possible to allow setting the arbitrary extension for the template file? Strictly speaking, a template file is not in HTML format and "html" extension is invalid.

[rla]

Hello @usr345, did you try to set the extension option? :) See options documentation: https://github.com/rla/simple-template#api