Avoid shell escaping for string arguments in `-D` options

[JanisErdmanis]

Currently, passing string arguments via -D options requires awkward escaping:

appbundler build . --build-dir=build --selfsign -Dbundler=\"juliaimg\" -Djuliaimg_selective_assets=true -Djuliaimg_sysimg=[\"QMLApp\"]

It would be better if -D options could be written directly as -Dbundler="juliaimg" or even -Dbundler=juliaimg, with lists of strings written as -Djuliaimg_sysimg=["QMLApp"].