CLI datasets

[timothyrenner]

Adds the ability to inject datasets into the script via command line options:

svl script.svl --dataset haunted=haunted_places.csv

would make haunted_places.csv available as a dataset in the SVL script without explicitly declaring it. Also, DATASETS is now fully optional.

[coveralls]

Pull Request Test Coverage Report for Build 119

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 98.204%

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Totals
Change from base Build 113:	0.03%
Covered Lines:	328
Relevant Lines:	334

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💛 - Coveralls