Support for 64x48 display size

[jpoikela]

This PR adds support for (a bit rare) 64x48 display size, for example https://www.wemos.cc/en/latest/d1_mini_shield/oled_0_66.html.

The change adds one else if in the initialization sequence, for size 64 x 48.

Limitations: I don't have access to other 64x48 displays, so I don't know of any.

The included example has been tested on WEMOS D1 mini v.3.1.0 with OLED 0.66 Shield v2.1.0, the only modification from 64x32 example being the display height.

If there is any chance to get this merged (I know Adafruit does not have this size), I'd be happy to also update the README file.

[jpoikela]

I noticed there are old PRs with the same goal:

• https://github.com/adafruit/Adafruit_SSD1306/pull/132: old but still open (should be closed without merge)
• https://github.com/adafruit/Adafruit_SSD1306/pull/136: old, closed and moved to #188
• https://github.com/adafruit/Adafruit_SSD1306/pull/188: there are more changes than I'd like.. after 64x32 support was merged, adding 64x48 is as trivial as this PR