Allow boolean properties to be serialised to more sensible strings

[jordangray]

Allow boolean properties to be serialised to "yes" or "no"—or even to custom strings—which would look better under most circumstances.

[jordangray]

I had to do this with JSON.NET recently and it wasn't fun. Would be cool to find an simpler way of achieving this. For example:

[Excel(trueValue="yes", falseValue="no")]

Or:

[ExcelBoolean("yes", "no")]

[jordangray]

Resolved by 013b619. Have added TrueValue and FalseValue to ExcelColumnAttribute.