Add sequence file generation

[leshane]

Make it possible to generate sequence files from the SeqParse simple, common format. File formats:

• GenBank
• FASTA
• GTF
• GFF

[leshane]

@jjtimmons - Just leaving this here. No urgency, but curious if you what you think about:

1. Extending this library or building a new library for generating sequence files
2. Rough estimate of time to implement

[isaacguerreir]

Made a PR #4 to start a conversation regarding file generation from Seq interface.