Heel Sequence Addition

_Note for Bernie: Pull this before pulling Rover Variant as it is chronologically prior._

Addition of two parameters, --senseheelseq and --antisenseheelseq that allow heel sequences to be added to the forward and reverse primer sequences universally.

Case of arguments supplied to the parameters will be maintained from input to output i.e. 'aaAa' will be aaAaSEQUENCE.