Show slab usage in memory meter

Slab is a kind of kernel memory which includes reclaimable usage. So it should be shown in different color.

https://github.com/hishamhm/htop/issues/67#issuecomment-49420284

Why I think slab usage should be separated is:

- It is just helpful if kernel, special mem usage is distinguished from user land one.
- Large reclaimable slab usage caused problem in production server monitoring, finally. [see newrelic discussion](https://discuss.newrelic.com/t/memavailable-vs-memfree/12760)
- Latest code of `free` command from procps doesn't includes slab usage in "used" (actually shown as cache == page cache + slab). [commit](https://gitorious.org/procps/procps/commit/6cb75efef85f735b72e6c96f197f358f511f8ed9)

Many thanks :)