Userland: Add an md command

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 ^)

Userland/md.cpp

@@ -0,0 +1,44 @@
+#include <AK/String.h>
+#include <LibCore/CFile.h>
+#include <LibMarkdown/MDDocument.h>
+#include <stdio.h>
+
+int main(int argc, char* argv[])
+{
+    const char* file_name = nullptr;
+    bool html = false;
+
+    for (int i = 1; i < argc; i++)
+        if (strcmp(argv[i], "--html") == 0)
+            html = true;
+        else
+            file_name = argv[i];
+
+    auto file = CFile::construct();;
+    bool success;
+    if (file_name == nullptr) {
+        success = file->open(STDIN_FILENO, CIODevice::OpenMode::ReadOnly, CFile::ShouldCloseFileDescription::No);
+    } else {
+        file->set_filename(file_name);
+        success = file->open(CIODevice::OpenMode::ReadOnly);
+    }
+    if (!success) {
+        fprintf(stderr, "Error: %s\n", file->error_string());
+        return 1;
+    }
+
+    auto buffer = file->read_all();
+    dbg() << "Read size " << buffer.size();
+
+    String input { (char*)buffer.data(), buffer.size() };
+    MDDocument document;
+    success = document.parse(input);
+
+    if (!success) {
+        fprintf(stderr, "Error parsing\n");
+        return 1;
+    }
+
+    String res = html ? document.render_to_html() : document.render_for_terminal();
+    printf("%s", res.characters());
+}