Support for differential Output

I was trying to make the differential Output work by loading the current patch for the VS1053 and using the following settings afterwards:
```
// Enable Mono Output
musicPlayer.sciWrite(VS1053_REG_WRAMADDR, 0x1e09);
musicPlayer.sciWrite(VS1053_REG_WRAM, 0x0001);

// Enable differential output
uint16_t mode = VS1053_MODE_SM_DIFF | VS1053_MODE_SM_SDINEW;
musicPlayer.sciWrite(VS1053_REG_MODE, mode);
```
Unfortunately this didn't work and took me quite a while to realize that startPlayingFile is doing
```
// reset playback
sciWrite(VS1053_REG_MODE, VS1053_MODE_SM_LINE1 | VS1053_MODE_SM_SDINEW);
```
Is there a reason for this? IMHO startPlayingFile shouldn't mess with the MODE register.

Start at startPlayingFile, especially the reset playback write to VS1053_REG_MODE, and compare it with the caller's differential-output configuration. Determine how playback can retain the requested MODE settings, then verify that differential output remains enabled when a file starts on the VS1053 hardware.

Written by the indexing model from the issue text.