Use yaml via -y to support additional Biosample attributes

[Julian Zhou]

1) For ConvertDb.py genbank, it could be very useful to allow support for additional Biosample attributes that are currently not specified through the command line options by supplying them in the yaml file via -y. Currently, supplying the yaml file is just another way to supply attributes through the command line (as opposed to supporting additional attributes).

2) On a related note, the current tutorial could be clearer in stating that the -y option does not support additional, unspecified Biosample attributes.

[Jason Vander Heiden]

In 5bbf182. Changed the behavior of the -y argument in the genbank subcommand. This argument is now featured to sample features only, but allows for the inclusion of any BioSample attribute. Added clarifications to the docs.