Initial implementation of -- shell commands
Initial implementation of -- shell commands

Re #29, one can now give an arbitrary string after the -- pseudo-arg and
it will be turned into an anonymous "remainder" task/command.

SamuelMarks/fabric,tolbkni/fabric,ericholscher/fabric,kxxoling/fabric,askulkarni2/fabric,tekapo/fabric,getsentry/fabric,xLegoz/fabric,bitmonk/fabric,opavader/fabric,cgvarela/fabric,pashinin/fabric,bitprophet/fabric,kmonsoor/fabric,simon-engledew/fabric,rbramwell/fabric,likesxuqiang/fabric,sdelements/fabric,amaniak/fabric,fernandezcuesta/fabric,raimon49/fabric,pgroudas/fabric,akaariai/fabric,ploxiln/fabric,haridsv/fabric,felix-d/fabric,itoed/fabric,elijah513/fabric,StackStorm/fabric,cmattoon/fabric,rodrigc/fabric,jaraco/fabric,bspink/fabric,hrubi/fabric,MjAbuz/fabric,rane-hs/fabric-py3,TarasRudnyk/fabric,mathiasertl/fabric,qinrong/fabric