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.

cheatsheets/ncat

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+# Connect mode (ncat is client) | default port is 31337
+ncat <host> [<port>]
+
+# Listen mode (ncat is server) | default port is 31337
+ncat -l [<host>] [<port>]
+
+# Transfer file (closes after one transfer)
+ncat -l [<host>] [<port>] < file
+
+# Transfer file (stays open for multiple transfers)
+ncat -l --keep-open [<host>] [<port>] < file
+
+# Receive file
+ncat [<host>] [<port>] > file
+
+# Brokering | allows for multiple clients to connect
+ncat -l --broker [<host>] [<port>]
+
+# Listen with SSL | many options, use ncat --help for full list
+ncat -l --ssl [<host>] [<port>]
+
+# Access control
+ncat -l --allow <ip>
+ncat -l --deny <ip>
+
+# Proxying
+ncat --proxy <proxyhost>[:<proxyport>] --proxy-type {http | socks4} <host>[<port>]
+
+# Chat server | can use brokering for multi-user chat
+ncat -l --chat [<host>] [<port>]