Test piping through non-R parsers (/exex/{gb2gff,seq-len}) on non-linux systems

[thackl]

read_seqs(".fna/.gbk/.gff") and read_gbk() use non-R parsers to preprocess the data before reading them into R. This is just done by turning the file argument provided to the read_* function ("foo.fna") into a connection ala pipe("/path/to/seq-len foo.fna"). This should work on linux systems, but probably not on Windows, and I'm not sure about MacOS...