Add additional MakeDb options for junction alignment information

[Jason Vander Heiden]

We should consider adding some additional alignment output to MakeDb concerning the start/end positions of V, N/P and D regions.

VDJTools requires these positions as input, which I made a conversion script for here, but we might be able to simplify this for people. I'm not entirely certain what the use cases would be, other than VDJTools support, so we should maybe figure that out.

The only bit that is not intuitively obvious is finding the end of the V-segment inside CDR3 (V_GERM_LENGTH_IMGT - 312), so maybe something like V_JUNCTION_LENGTH and J_JUNCTION_LENGTH would be worth including. Not sure.

[Jason Vander Heiden]

A lot this was included in the --junction argument to MakeDb-imgt. The rest is part of the AIRR format.