Add support for <DEL> in bcf-consensus

The code now reads <DEL> in the ALT allele as a valid exception.

*NOTE* Bug in mask_region functionality. Apparently this happens when
a variant (a deletion) removes sequence from the next FASTA line.
This results in masking happening one line too far. Second
test_consensus fails now.
New -c/--chain option in bcftools-consensus

This option saves the transformation of the reference to the
consensus sequence in a chain file that can be used with the
liftOver tool.
Bug fixes in consensus

- the position of a symbolic allele marks the coordinate *before* the event
- mask_region coordinates did not take into account indels
- updated tests which seemed wrong.

Note: This needs further review, more bugs likely!