Strengthen error handling for new files

I got some new FASTA files that broke my converter because their
sequence description headers did not comply to the standard I found
in the previous sample files I had been given.
Add description of intended fields

Based on a conversation with Bochao, I added a description of what
fields are to be filled in the TAB files, and roughly how to calculate
them.
Remove ugly debug messages

I removed the debug messages that I used when I was first adopting
argparse, and putting in CL options.
Refactor ClipRecord to accomodate discrepancies

I refactored the __init__ method of ClipRecord to read in attributes
from the FASTA description line in a separate method, which is called
early in __init__, this will allow, in the future, for me to plug in
different code to read in attributes from those lines.
Added option to generate TAB file of germlines

I added to fasta2tab the option to generate a separate TAB file just
from the germline sequences.
Allow for germline sequence lookup in masking

I modified the argparse to make the mask option a string, and one of
several options.
Notably, one of the options is to look up the sequence of the germline
from which this sequence is descended and put that sequence into the
GERMLINE_GAP_D_MASK field.
Currently, no method is implemented to proprerly mask the D region of
the germline sequence, so the unmasked germline sequence is used.
Refactor D masking method to use separate function

I refactored the method ClipRecord.d_mask to employ a separate function
to do the D masking.
It is just a little cleaner.
This will also be helpful as I revise the way I mask the D region.
Refactor header row delimiters to class attributes

I removed the hardcoded header row delimiters from the
ClipRecord._set_from_header method into class attributes.
This will make it easier for me to accomodate alternate header row
schema.
Refactor FASTA description parsing

FASTA description parsing into key-value pairs is now done by an
external function, rather than being built into the
ClipRecord._set_from_header method.
This will be helpful to accomodate other header formats.
Make germline lookup work on both file formats

I changed the way that 'CLONE' and germline related attributes are
calculated and stored in order to make fasta2tab compatible with both
types of files I have encountered so far.
Improve error handling in germline lookup

Now if the germline specified by a sequence in the 'CLONE' field is not
found among the germlines processed from the file, a helpful error
message is printed, showing the name of the clone that could not be
found.