As @dbohdan suggested in #2, we should implement stdin as an input, and stdout as an output, and make them default.

This can be confusing for someone with poor CLI experience (me) because if you just type "htmd" it will wait for input and not prompt you anything. But this is how other cli tools on *nix do, this will allow us to interact with other tools, such as grep, cmark, etc.

htmd < hello.html

htmd < hello.html | grep "heading"

cmark < hello.md | htmd

Fs input and output still work the same way, but stdout as the output won't work when your input has multiple files.