Add downloading of custom datasets to data_fabfile.py

[brainstorm]

Often private companies offer targeted sequencing data, adapters or other "copyrighted" material that can only be downloaded from their respective website, often hardly accessible by automatic means.

This addition would allow to define a custom task to download data from a host that holds all this manually downloaded privative data (protected by password).

That way, this kind of data must be manually mantainted once, for potentially many instances that use the files.