Added option for blast2graph.py to split FASTA into connected components

The graphs produced from the set of top BLAST hits are frequently sparse and
have many connected components that can be analyzed independently for
convenience. blast2graph.py now has a --fasta option that takes the FASTA file
used to generated the given set of BLAST results, splits the FASTA records by
connected components, and then prints one FASTA file for each component. These
separate FASTA files can then be analyzed using a variety of multiple sequence
alignment, phylogenetic, or other software.

Signed-off-by: trgibbons <ted.codes@gmail.com>