Added additional column to monitor OOM killer score of each process

Hi Hisham,

Thanks so much for _htop_ - I've been using it for a few months and am really grateful for it. Today I ran into a problem that it didn't solve, and thought I'd have a go at adding what I needed.

I have a small VM where the Linux OOM (out-of-memory) killer kills MySQL every few weeks. Looking into this I found I could read the ```oom_score``` of a process from ```/proc/nnnn/oom_score```. I wanted to review the scores of all the running processes before I started changing things, but couldn't find that column within _htop_. I've added a new column called "OOM" to display this information.

I've adjusted the column width in the same way as the PID column but am not sure that was the best thing to do. If you don't like that could you help me improve it?