allow expresions when reading named parameter from command line

in the strategy file:

let { trac ?= False
    ; verb ?= False
    ; bit ?= 3
    ; dim ?= 9
    ; enc ?= Simple
    ; dom ?= Arctic

but then this does not work:

time pure-matchbox +RTS -N8 -RTS --print-proof -S strat/dp-matrix-single.strat data/Zantema/z001.srs verb=True trac=True dim=5 bit=5 dom=Natural enc="Wallace DFS"

last argument is not parsed, but replaced with default (silently, was noch schlimmer ist)