Custom --help for command line install

QtIFW allows us to supply custom arguments to the installer:

installer.exe [options] command <key=value>

End users won't know these parameters exist without external documentation.

installer.exe --help should be extended to :

• Minimally, allow us to print custom translatable text. Insert it somewhere in the existing default help message

• Alternatively, replace installer.value() with something self documenting, like all major CLI parsing libraries do