Added support for initial command, so it can be run as `%xterm <command>`

[robintw]

This PR adds a new feature where the xterm magic can take an argument of a command to run as soon as the terminal is opened.

So, you can now run %xterm ls to open a xterm and immediately run the ls command. Very helpful for interactive documentation in a jupyter notebook, where you need to explain/document the running of terminal commands too.