Be explicit and say that the program only takes input from STDIN.

Don't print the lines exactly as they come in. Instead run them through
`unxhtmlify_string` and then print them out. This gives us the
unXHTMLified version of stdin.

The word "line" didn't really make sense since the function doesn't care
about lines. I started with that naming because it made sense in terms
of the implementation, or of the reading of a buffer and unxhtmlifying.

Use regex to search for the slash bracket of a self-closing tag. Tests
pass, wow. Was not expecting that. I think I need to add more test
cases.

Expose some failures with more test cases.

Do a string replacement to delete " /" from the input.

Create a function stub that will remove the slash from self-closing HTML
tags. Add a basic test that for now just tests the <br /> element.