Added interesting feature to Iron Man Jump

Hello Nickey,

In case you don't know who this is, this is Donald Leung from Kellett School who you taught code to last year.  I am here to announce an interesting tweak I have added to your Iron Man Jump.

If you download my version of your webpage and play it, you will realise that the gravity increases over time.  If you like the tweak that I made to your game, please merge commits.

By the way, I have this GitHub account because I went to an ECA called the JA Company Programme and our "company" had to have a website.  Check it out here:

http://kellettjainnovision.github.io

Also, I have recently changed the whole layout of my website (again):

http://donaldkellett.github.io

Cheers,
Donald