Add another check function: check_interrupts

[inqrphl]

This check reads the /proc/interrupts file , parses it and puts each interrupt count in its own entry.

An interrupt is identified by a number or by an acronym. After that comes the counts. Almost all of the interrupts have CPU specific counters, I only found two which are globally counted. At the end there are some extra columns detailing the interrupt name, type, vector, driver etc. I parse these and expose them as attributes to check/filter for.

Wrote a test function for checking the /proc/interrupts output parser. I can add additional outputs from different systems and test for them.

Also wrote the markdown documentation for the check.