Add write support for local symbol table append.

[tgregg]

After read support for local symbol table append is added (see #22 ), write support should be added.

This should be triggered via IIonWriter.Flush. When that method is called, all buffered data is flushed. If any symbol tokens are subsequently written, their symbols will be added to the stream via local symbol table append syntax.

Note: IIonWriter.Finish should flush all buffered data and reset the symbol table context, such that a brand new local symbol table will precede any subsequent values that contain symbol tokens.