add FastaFormat#first_name method

One method I wish existed, but didn't appear to before this pull request, is simply to return the first word of the fasta defline, minus any other parsing.

I realise this is rather trivial, but seems to be very often used, e.g. by tab separated BLAST output, but there doesn't appear to be any method for it. Am I overlooking it, or is it too trivial?

I'm not sure if there is a standard way saying "first_name" - I just made that up.