Method to mirror the display

Fix warnings when compiling against GCC 5.4

correct type to avoid compiler warning

Allow for use of either 128x64 or 128x32 displays

Support for custom font table look-up function

Fixes the following warnings when compiling using GCC 5.4:

    CXX OLEDDisplayUi.o
    ./OLEDDisplayUi.cpp: In member function 'void OLEDDisplayUi::drawFrame()':
    ./OLEDDisplayUi.cpp:284:48: error: 'y1' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	    x *= dir; y *= dir; x1 *= dir; y1 *= dir;
						    ^
    ./OLEDDisplayUi.cpp:284:37: error: 'x1' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	    x *= dir; y *= dir; x1 *= dir; y1 *= dir;
					 ^
    ./OLEDDisplayUi.cpp:284:26: error: 'y' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	    x *= dir; y *= dir; x1 *= dir; y1 *= dir;
			      ^
    ./OLEDDisplayUi.cpp:284:16: error: 'x' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	    x *= dir; y *= dir; x1 *= dir; y1 *= dir;
		    ^
    ./OLEDDisplayUi.cpp: In member function 'void OLEDDisplayUi::drawIndicator()':
    ./OLEDDisplayUi.cpp:393:35: error: 'y' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	   this->display->drawFastImage(x, y, 8, 8, image);
				       ^
    ./OLEDDisplayUi.cpp:393:35: error: 'x' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    ./OLEDDisplayUi.cpp:387:7: error: 'posOfHighlightFrame' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	   if (posOfHighlightFrame == i) {
	   ^
    cc1plus: some warnings being treated as errors

Fix the following warnings, seen when compiling with GCC 5.4:

    CXX OLEDDisplay.o
    ./OLEDDisplay.cpp: In member function 'void OLEDDisplay::drawStringInternal(int16_t, int16_t, char*, uint16_t, uint16_t)':
    ./OLEDDisplay.cpp:381:10: error: enumeration value 'TEXT_ALIGN_LEFT' not handled in switch [-Werror=switch]
       switch (textAlignment) {
	      ^
    ./OLEDDisplay.cpp: In member function 'void OLEDDisplay::drawInternal(int16_t, int16_t, int16_t, int16_t, const char*, uint16_t, uint16_t)':
    ./OLEDDisplay.cpp:722:13: warning: unused variable 'yScreenPos' [-Wunused-variable]
	 int16_t yScreenPos = yMove + yOffset;
		 ^
    ./OLEDDisplay.cpp: In member function 'void OLEDDisplay::drawXbm(int16_t, int16_t, int16_t, int16_t, const char*)':
    ./OLEDDisplay.cpp:361:19: error: 'data' may be used uninitialized in this function [-Werror=maybe-uninitialized]
	     data >>= 1; // Move a bit
		       ^
    cc1plus: some warnings being treated as errors

Fix platforms in library.json

fix typo in integer type (causes compile error)

`warning: narrowing conversion of '128' from 'int' to 'const char' inside { } [-Wnarrowing]`

Closer to middle vertical align

Make OLEDDisplay.display() an abstract function

Implement setLoadingDrawFunction. Fixes #102