function setExtMap(map, extname, val) {
    if (extname && extname.indexOf('.') === 0) {
        extname = extname.substring(1);
    }
    if (!extname) {
        return;
    }
    map[extname] = val;
}
Quick helper to set a value on one of the extension maps. Ensures there is no
dot at the start of the extension.
@ignore
@param map - The map to set on.
@param extname - The extension (or key) to set.
@param val - The value to set.