text: Title: [ENH] to_fasta as part of janitor.biology
  Body: `janitor.biology` could do with a `to_fasta` function, I think. The intent here would be to conveniently export a dataframe of sequences as a FASTA file, using one column as the fasta header.
  
  strawman implementation below:
  
  ```python
  import pandas_flavor as pf
  from Bio.SeqRecord import SeqRecord
  from Bio.Seq import Seq
  from Bio import SeqIO
  
  @pf.register_dataframe_method
  def to_fasta(df, identifier_column_name, sequence_column_name, filename):
      seq_records = []
      for r, d in df.iterrows():
          seq = Seq(d[sequence_column_name])
          seq_record = SeqRecord(seq, id=d[identifier_column_name], description="", name="")
          seq_records.append(seq_record)
      SeqIO.write(seq_records, filename, format="fasta")
  ```