[unused_configuration_options] One tuple files breaks

[pbrudnick]

Bug Description

The rule unused_configuration_options breaks on analyzing a config file like:
https://github.com/erlang/otp/blob/master/lib/common_test/test_server/ts.unix.config

Using a .config file with more than one tuple does not break.

To Reproduce

Create a .config file with just a single tuple.

Expected Behavior

Should be valid as multi-tuple and proplists config files.

rebar3 Logs

unused_configuration_options:analyze/3 failed with Error function_clause 
Stack: [{unused_configuration_options,'-config_keys/2-lc$^0/1-0-',
            [{unix,
                 [{telnet,"belegost"},
                  {username,"telnet-test"},
                  {password,"tset-tenlet"},
                  {keep_alive,true}]}],
            [{file,
                 "/Users/pablo/dev/AdRoll/rebar3_hank/src/rules/unused_configuration_options.erl"},
             {line,69}]},