[ToDo] Support splitting `:`-delimited sequences into multi-FASTA

[jscgh]

ColabFold uses : to denote inter-protein chainbreaks. It would be useful to automatically split such input into separate FASTA entries for better software compatibility.

Example input:

NLYIQWLKDGGPSSGRPPPS:NLYIQWLKDGGPSSGRPPPS

Expected output:

>A                                                                           
NLYIQWLKDGGPSSGRPPPS
>B                                                                           
NLYIQWLKDGGPSSGRPPPS

[tlitfin]

First priority for this would be to support --aa-string input (common use case) and can add support for fasta later if it adds complexity (likely much more rare).