Clear the colon when clearing the display

[mortenfyhn]

Hi, I've been testing out your library and I found that if I enable the colon, then clear the display, the colon persists.

For instance, this sketch will result in the colon staying lit:

#include <SevenSegmentTM1637.h>

void setup()
{
    auto display = SevenSegmentTM1637{7, MISO};

    display.setColonOn(true);
    display.print(1234);

    delay(1000);

    display.clear();
}

void loop() {}

I think the intended behaviour here is to clear everything when clear() is called, which is what I've suggested in this patch.