Add 'clear' command to clear terminal.

[jghibiki]

I was playing with it and the terminal gets very cluttered very quickly, and my eyes are bad. A clear command might be helpful for increasing readability of long terminal sessions.

For reference here is an article (to save you time) on different ways to clear the screen in cpp.

[dekarrin]

@jghibiki I agree with your points on this, #30, and #31 but I won't be able to get to them for some time yet as I am currently occupied with creating the moe-serifu-circle/website.

If you're interested in seeing these changes before then you totally have the go-ahead to make it so. I'd imagine the majority of the work would be done in src/output/output.cpp