Add flag to export multiple paths for fasta

[dkhofer]

cargo run -- init
cargo run -- import --fasta fixtures/simple.fa
cargo run -- update --fasta fixtures/aa.fa --start 3 --end 5 --region-name m123 --new-sample test1\n
cargo run -- update --fasta fixtures/aa.fa --start 15 --end 20 --region-name m123 --sample test1 --new-sample test2\n
cargo run -- export --fasta out.fa --all-paths -s test2