Adds Blacklist/Whitelist support to snmp plugin

Adds two options to snmp plugin Data Section:
	- Ignore -> A List of string containing patterns to blacklist
	- InvertMatch -> A Boolean value to tell if using blacklist or
	  whitelist (true is whitelist, false (default) is blacklist)

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>