Added mkdir, updated coreutils.txt accordingly

I tried to follow the general structure of the other files as best as possible. There are just a couple of things that I wanted to mention:

* Spacing on line 70 and 76 to improve readability. I would have liked to have the if blocks as close to the function calls as much as possible, but it just looked a bit cluttered to me. 
* Using log.Fatalln to report an error and exit. Its output message is a little more detailed than the regular mkdir error output. It displays the current date and time before describing the actual error. I like that it is more detailed, but not so much since it isn't emulating mkdir **exactly**. 
* I added a usage_text to be displayed when mkdir is called without any arguments or options, again to emulate mkdir as much as possible. It also reflects the current options that are available. 

Please let me know what you think, especially if I should change anything. Thanks!