Un-hard-code I2C address

[jamwaffles]

Section 8.1.5) a) in the datasheet states that the display address can be 0x3C or 0x3D, chosen with the D/C pin.

It can now be configured using the .with_i2c_addr() method on the Builder. Example:

let mut disp = Builder::new()
        .with_size(DisplaySize::Display128x32)
        .with_i2c_addr(0x3d)
        .connect_i2c(i2c);

Closes #12.

@scowcron This is where the builder pattern starts making sense to me; it feels more Rustic to chain a bunch of calls together to configure the object. Feels a bit like chaining iterator methods.

[scowcron]

Yep, I see where you're going with it. Seems a little bit odd that you can set an i2c address which will then go unused if you run the SPI route, but it's not really a big deal, and I suspect it will be optimized out with the --release flag anyway if someone decides to do that.

[jamwaffles]

Seems a little bit odd that you can set an i2c address which will then go unused if you run the SPI route

True, but it's explicitly named *_i2c_* so should hopefully make sense to people using it.

I suspect it will be optimized out with the --release flag anyway

I suspect also, but it would be good to confirm this at some point.