Modify header handling to tolerate thin headers

Thin headers have no key value pairs in the description, merely a
single string which is assumed to be the SEQUENCE_ID.
Improve error handling

Specifically strangely formatted headers in germline records are more
lazily handled.
Modify gitignore to ignore test data files
Refactor ClipRecord masking into separate function
Refactor tabfile writing into separate function

This sets the stage for me to conveniently add another function to
do multiple fasta files into a single tabfile.
Add functionality to combine multiple fasta files

Now I can merge multiple fasta files into a single tab file,
or still do them separately.