Provide default serial port settings for each supported machine

Currently, Plover uses the same set of default serial port settings for all machines. Since different machines likely require different serial port settings, it makes sense to provide different default settings for each supported machine. This will hopefully smooth out the experience of using a new machine. The serial port settings should remain user configurable, but the defaults should be sensible. This could be accomplished by adding default configuration parameters to each machine type and then reading those parameters when generating the default configuration file. This blueprint was originally suggested by Hesky.