Add BTC Cost to hashmarket

This code works perfectly if I just paste it into the console. I am thinking about making it a bookmarklet to make it easier. I am submitting the pull request because you will likely know how to make it work with your script. I am not familiar with the way tampermonkey works exactly. 

It says `Uncaught TypeError: Cannot read property 'innerText' of undefined` which I am assuming because it is executing the code before the table is loaded. I just don't know how to make it wait an adequate amount of time.