Add support for drive strength, BCD and VBUS_Sense

[flatmush]

I'm making a games machine as documented here:
https://forum.odroid.com/viewtopic.php?t=33936

I've been programming the FT231X using your program which I've found very useful.

Since this device is aiming to be low power I needed to use the VBus_sense feature, so I had to implement that.
I'm using the Battery Charge Detect feature to determine when I can charge the battery at full speed so I implemented that.
I also added the options for drive strength since I needed to tweak those to get the chip working correctly with my reset line.

Hope you find these changes useful.

[richardeoin]

Thanks for the PR! I'm not sure how generally useful setting bcd_enable or vbus_sense_alloc is, so I'm not going to merge that for now. I think the driver strength control went in as part of #21

[rkralovic]

If bcd_enable is not set, setting CBUS to BCD_Charger or BCD_Charger# makes the pin always tri-state. (I just figured it out after trying to use BCD_Charger and then implementing the bcd_enable setting logic myself.)

In my opinion, the current behavior (not allowing the user to set bcd_enable) is a bug, since it makes it impossible to use BCD_Charger output on CBUS pins.