Refactor for better SW SPI only support

[caternuson]

For #81, but also motivated by:
adafruit/Adafruit_DotStar#50

Essentially just some preproc foo.

This simple DotStar example runs as expected on a Trinket 3V (PID 1500).

#include <Adafruit_DotStar.h>

#define NUMPIXELS  10
#define DATAPIN    2
#define CLOCKPIN   1
Adafruit_DotStar strip(NUMPIXELS, DATAPIN, CLOCKPIN, DOTSTAR_BGR);

void setup() {
  strip.begin();
  strip.fill(0xADAF00);
  strip.show();
}

void loop() {
}

And testing a BME280 wired to hardware SPI on a QT PY M0 still works (didn't break HW SPI check):

BME280 test
-- Default Test --

Temperature = 24.99 °C
Pressure = 1006.83 hPa
Approx. Altitude = 53.57 m
Humidity = 57.60 %

Temperature = 25.24 °C
Pressure = 1006.87 hPa
Approx. Altitude = 53.28 m
Humidity = 57.52 %

Temperature = 25.28 °C
Pressure = 1006.85 hPa
Approx. Altitude = 53.40 m
Humidity = 57.43 %

[caternuson]

@ladyada for review