Ready for MERGE:  Add command to import preprocessed data

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

I think this should be called ```filepath_type``` which is different from ```filetype``` in other contexts. Also, similar comment here as with the ```--params_table``` option.  I wrote a util for retrieving the filepath_types (see util.py's get_filepath_types), which will allow you to do something like:
```python
from qiita_db.util import get_filepath_types
# snip ...
@click.option('--filetype', help="Describes the contents of the input files", type=click.Option(get_filepath_types().keys())
```