Ready for MERGE:  Add command to import preprocessed data

Tests will not pass until @josenavas issues a new pull request. 

It would be nice to have this as a ```click.Choice``` option where it shows you what ```preprocessed_params_``` tables are available for this option.  You can have a look at #129's implementation (relevant files: qiita_db script, util.py, test_util.py), which uses postgres's ```information_schema```