Support for file extensions other than .md

[imschutz]

Hi,

In current version, I noticed that usage of the pymarkdown tool is limited to files with extension .md.

That is due to

pymarkdown/pymarkdown/main.py

Line 212 in 0476c41

return path_to_test.endswith(".md")

I understand that this may be a default, but markdown can technically be used in any text file. I personally have a use case where the files are forced to have a different extension (due to a toolchain that renders the files), but for the most part they are still markdown and could very much use some linting :)

Expanding the existing function to checking against a list of extensions should be easy enough - if that would be a viable solution, I could perhaps propose a PR.

Thoughts?