Using int was a mistake. This patch changes String, StringImpl,
StringView and StringBuilder to use size_t instead of int for lengths.
Obviously a lot of code needs to change as a result of this.
This command uses LibMarkdown to parse and render Markdown documents
either for the terminal using escape sequences, or to HTML. For example,
you can now do:
$ md ReadMe.md
to read the Serenity ReadMe file ^)