The following website provides a tool that will translate markdown into HTML:
Once you convert the file to HTML, there are a number of command line tools to use to view the file. Using a test file that contains markdown formatted-text, I found the following worked nicely.
$ wget http://daringfireball.net/projects/downloads/Markdown_1.0.1.zip
$ unzip Markdown_1.0.1.zip
$ cd Markdown_1.0.1/
$ ./Markdown.pl ~/testfile.markdown | html2text
html2text is one of many tools you can use to view html formatted text from the command line. Another option, if you want slightly nicer output would be to use lynx:
$ ./Markdown.pl ~/testfile.markdown | lynx -stdin
If you are an emacs user, someone has written a mode for markdown which is available here: http://jblevins.org/projects/markdown-mode/. This provides nice syntax highlighting as can be seen in the screenshot on that website.
All of these tools should be available for slackware.