Add option to exclude alarms and usage elements

[martialblog]

This adds an --exclude flag, that excludes alarms or usage from the check results. Can be used multiple times and supports regular expressions.

Example:

$ ./check_vmware_nsxt.py --exclude "LOW" --exclude "My Alam .+ Foobar" --mode alarms

See #10

[martialblog]

@sklaes Feedback/Testing is welcome