Our package member list to markdown-fragments readable stream.  It
reads our srcs directory and outputs a markdown-fragments friendly list
of all the packages and members in said directory.
@param dirToScan {String} - Dir to scan for list.
@param options {Object|undefined} - Optional.  Readable stream options.
@constructor {PackageMemberListReadStream}
function PackageMemberListReadStream(dirToScan) {
    sjl.throwTypeErrorIfEmpty('PackageMemberListReadStream', 'dirToScan', dirToScan, String);
    this._pathToRead = dirToScan;
    Readable.call(this, {
        encoding: 'utf8',
        objectMode: false,
        highWaterMark: 100000,
    });
}