Get the map of the file extname with processor name.

@inner
@param {Object} processors the builtin processors, the key is processor name,
the value is the processor config information.
@return {Object}
function getFileExtnameAssociatedProcessor(processors) {
    let result = Object.keys(processors).reduce(
        (lastValue, processorName) => {
            let processor = processors[processorName];
            let extnames = processor.extnames || [];
            if (!Array.isArray(extnames)) {
                extnames = [extnames];
            }
            addFileExtnameAssociatedProcessor(extnames, processorName, lastValue);
            return lastValue;
        },
        {}
    );

    Object.keys(result).forEach(
        k => sortDefaultProcessors(result[k], processors)
    );
    return result;
}