Add `crystal spec --list-tags` #872

Job	Run time
Build interpreter	15m 54s
Test Interpreter	5m 38s
Test std_spec with interpreter (0)	2m 28s
Test std_spec with interpreter (1)	6m 24s
Test std_spec with interpreter (2)	2m 34s
Test std_spec with interpreter (3)	3m 19s
	36m 17s

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