Add ssd1306_clear_square function

[maro159]

It is expansion of idea introduced in PR #11
Funtion to clear area defined as square instead of single pixels may be simpler to use in some cases, at least in my opinion.

Also corrected single typo in ssd1306.h file

See an example:

#define FONT font_8x5
#define FONT_WIDTH (FONT[1]+FONT[2])
#define FONT_HEIGHT (FONT[0])
#define OLED_WIDTH 128
#define OLED_HEIGHT 64
#define OLED_ADR 0x3C
#define OLED_I2C i2c1

// initialization
static ssd1306_t oled;
ssd1306_init(&oled, OLED_WIDTH, OLED_HEIGHT, OLED_ADR, OLED_I2C);
oled.external_vcc = true;

// displaying something...

// clear one line of text 
ssd1306_clear_square(&oled, 0, 0, OLED_WIDTH-1, FONT_HEIGHT);

[daschr]

Sorry for the late reply. Indeed that's more comfortable.
Thank you very much!