splunk: add index to options

[pcoccoli]

Closes #859

[codecov]

Codecov Report

Merging #993 (20c54e3) into develop (fd80ec1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #993      +/-   ##
===========================================
+ Coverage    64.71%   64.73%   +0.01%     
===========================================
  Files          517      517              
  Lines        48873    48876       +3     
===========================================
+ Hits         31629    31639      +10     
+ Misses       17244    17237       -7     

Impacted Files	Coverage Δ
...dules/splunk/stix_translation/query_constructor.py	71.59% <100.00%> (+0.49%)	⬆️
...odules/splunk/stix_translation/query_translator.py	89.28% <100.00%> (+0.39%)	⬆️
.../tests/stix_translation/test_splunk_stix_to_spl.py	82.53% <100.00%> (+2.96%)	⬆️

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