added diskStats function and MIB

I've created a function to parse /proc/diskstats file. There is a MIB file, a parsing function itself and a configuration example. By default, local file is parsed. I provided some basic features like "arbitrary command execution" and "block device filtering by name".
"arbitrary command execution" means that you can use any commands (e.g. self-written script) which produces 14 fielded output (like diskstats file). "command" option is used to configure arbitrary command.
"block device filtering by name" means that only some output is processed. A line is processed if its second field matches an RE. RE can be set using "match" option.
Any option can be omitted. Only one copy of arguments (the first one) will be used.