Added cheatsheets for markdown, ncat, and sqlmap.

Some cheatsheets that I created for a project for CIS4930, Ethical
Hacking and Penetration Testing. On request of the professor, I will be
sending a pull request with these cheatsheets.
Merge pull request #136 from andrewjkerr/additional-cheatsheets

Added cheatsheets for markdown, ncat, and sqlmap.