expose mafft's --keeplength parameter for mafft-add

[mikerobeson]

The --keeplength parameter is useful, when the intent is to add fragments / primer sequences to an existing alignment. This will force the main alignment length to not change when adding primer sequences. When this parameter is not used, it is possible for the original alignment, to which the primers / fragments are being added, to change in size, i.e. the number of alignment positions. I've noticed a few cases were my final alignment, including the fragments / primers, shrunk in alignment length. This is problematic when trying to determine the exact alignment positions a primer is located in reference to a curated alignment, e.g. SILVA.

[gregcaporaso]

Sounds good @mikerobeson! We're putting this on our Approved but unscheduled board.