Fix for #59
Fix for #59

The golang JSON serializer errors out when it encounters numeric values
like +Inf, -Inf, and NaN.  This commit adds an option
-prune-invalid-json which simply drops the value if it is encounter
before sending the document to be indexed.  I figure it is better to
send no value in this case than do a conversion.

rwynn/monstache,rwynn/monstache