statemanager.startup_with_shell_off should be optional

I'm trying to use gdbgui with gdb-z80 backend on macOS. As such, gdb-z80 has nothing to do with Darwin. However, when statemanager detects that it runs on Darwin, it inserts extra arguments to gdb command line which happen to be unsupported by my specific version of gdb. This results in a very puzzling error message:

> The underlying gdb process has been killed. This tab will no longer function as expected.

It would be easier to diagnose such problems if the actual error messages from gdb were displayed instead of this enigmatic line.

I hacked this bit off for my purposes, but it would be nice if command-line options passed to gdb at start were explicit and configurable. I imagine any cross-platform debugging setup would run into a similar issue sooner or later. At least it should be possible to do so when custom gdb is specified with -g option.

**Platform details:**
* OS: Darwin
* gdbgui version 0.13.0.0
* gdb version (`gdb -v`): GNU gdb (GDB) 7.2.50.20101026-cvs
This GDB was configured as "--host=x86_64-apple-darwin17.5.0 --target=z80"
* browser [e.g. chrome, safari]: chrome