Support for multi-FASTA input

[apcamargo]

For large datasets, having a separate FASTA file for each input sequence can overwhelm the file system. Therefore, it would be useful if Pling! could accept multi-FASTA files.

From what I've seen of the codebase, this would likely require a significant restructuring (so feel free to disregard this suggestion!). One possible solution is to use a library like pyfastx to access sequences as needed, although this might result in a performance drop.