Added option to ignore missing library warning (-i)

Hey Dave. Nice to see you continue working on Nagios plugins :+1: 

Added an option "-i" to ignore the missing library warning. For basic checks (like ping) the libraries are not needed, the warning in the output is causing problems for the nagios output. 

Before:

    ./check_riak -H localhost -n ping 
    [warning] lib directory not found, third party commands will fail
    OK

With the patch:

    ./check-riak -H localhost -n -i ping 
    OK