Add accessibility support based on option flag

scripts/run_archiso.sh:
Add the `-a` option flag to add accessibility support on demand for `run_archiso`.

Not providing the specific braille chardev (hardware?) otherwise blocks running run_archiso with
`qemu-system-x86_64: -chardev braille,id=brltty: brlapi__openConnection: connect: No such file or directory`.

Fixes #77