enable direct-to-cache imports

[gregcaporaso]

To speed up import of very large data sets, we should be able to directly import data (e.g., a collection of fastq.gz files) to an artifact cache, without going through an Archive (.qza). This would avoid the zipping of the large dataset.

[Oddant1]

I moved this to the cli because this is actually totally possible in the Python API by doing the following:

cache = Cache(path)

art = Artifact.import_data(type, view)
cache.save(art)

[ebolyen]

@Oddant1, I created another issue for a slightly different problem: https://github.com/qiime2/qiime2/issues/730