Allow adding of complex metadata

Add option to add_metadata.py (--sc_pipe_separated),
to allow metadata field to be added as list of lists,
instead of just list of strings as done by --sc_separated.

This is useful when there is a one to many mapping (e.g. KOs to
KEGG Pathways). PICRUSt can use this metadata format when using the 
categorize_by_function.py.

Example:

L1_A;L2_B;L3_C|L1_A;L2_D
becomes
[[L1_A,L2_B,L3_C],[L1_A,L2_D]]