Add support for parsing /proc/mdstat

[vjt]

The /proc/mdstat file contains the current status of linux md devices, that are software RAID implementations provided by the kernel.

The /proc/mdstat file is produced by this kernel code, it has a plain-text format. Some examples are available in this wiki page.

Due to the nature of the format, it can be parsed with regular expressions. An example parser (in Ruby) is available here.

Ideally, the output of the parsing should be a Rust data structure that contains the information available as plain text.

Thanks