Force open files with extension other than .md

[johnd0e]

Currently frogmouth refuse to open anything except *.md.
It would be useful to allow other extensions as well.
For me it would enough something like --force command line option.

Related: #78

[spearman]

So I found out that you can add more file extensions in configuration.json (~/.config/textualize/frogmouth/configuration.json). You can also add an empty string "" and it will be able to browse markdown files without extensions.