stdout on creation of artifact cache entry

[gregcaporaso]

One little thing that would be nice is if the output summary from q2cli was a little different for cache entries that were created, relative to artifacts created as files. For example, I think (current behavior):

Saved FeatureTable[Frequency] to: /projects/microbiome/jgc-cache/:test_table
Saved FeatureData[Sequence] to: ./test_seqs.qza
Saved SampleData[DADA2Stats] to: /projects/microbiome/jgc-cache/:test_stats

could be clearer as something like (proposed behavior):

Added FeatureTable[Frequency] to cache /projects/microbiome/jgc-cache/ as test_table
Saved FeatureData[Sequence] to: ./test_seqs.qza
Added SampleData[DADA2Stats] to cache /projects/microbiome/jgc-cache/ as test_stats

[Oddant1]

I believe this was closed by #290