Support of custom dub init type

Command dub init now supports custom dub packages for argument `-t`.
$(CONSOLE
> dub init -n myPackage --format sdl -t custom-dub-init-dubpackage -- --foo=bar
)

Dub init will be invoked like before. The package `custom-dub-init-dubpackage`
contains a sub package `init-exec` which will be invoked afterwards to create a custom
package skeleton. Additional arguments could be passed e.g. `-- --foo=bar`.