Switch plFormat's accumulator to a plStringStream for better performance

I did some quick performance testing of plFormat, and discovered that it was unfortunately quite a bit slower than other formatters...  So I switched the output mechanism to a plStringStream instead of a list of buffers, and got much better performance (~1.5x-6x improvement, depending on the compiler).

For reference, these are my timings from 100k loops of a simple format test:
```
                                    GCC -O0    GCC -O3  Clang -O0  Clang -O3    VC12 Dbg    VC12 Rel

No-op                           :   1.48 ms    0.13 ms    2.24 ms    0.14 ms     0.98 ms     0.00 ms
static snprintf                 :  32.69 ms   31.57 ms   52.49 ms   30.65 ms   232.58 ms   120.21 ms
dynamic snprintf                :  63.10 ms   61.90 ms  118.53 ms   63.06 ms   497.40 ms   270.67 ms
plString::Format                :  47.72 ms   33.45 ms   82.54 ms   34.86 ms   465.15 ms   210.10 ms
plFormat (current)              : 225.26 ms   93.05 ms  360.92 ms   91.13 ms  3382.12 ms  1211.74 ms
plFormat (plStringStream)       :  86.36 ms   63.41 ms  104.45 ms   65.76 ms   571.67 ms   268.73 ms
```

The changes are non-trivial, so I did perform some basic regression testing, but any additional testing is also welcome.