function validate( opts, options ) {
	if ( !isObject( options ) ) {
		return new TypeError( format( 'invalid argument. Options argument must be an object. Value: `%s`.', options ) );
	}
	opts.slug = options.slug;
	if ( !isString( opts.slug ) ) {
		return new TypeError( format( 'invalid option. `%s` option must be a string. Option: `%s`.', 'slug', opts.slug ) );
	}
	opts.file = options.file;
	if ( !isString( opts.file ) ) {
		return new TypeError( format( 'invalid option. `%s` option must be a string. Option: `%s`.', 'file', opts.file ) );
	}
	return null;
}
Validates function options.

@private
@param {Object} opts - destination object
@param {Options} options - function options
@param {string} options.slug - public GitHub repository slug
@param {string} options.file - filepath
@returns {(Error|null)} error object or null

@example
var opts = {};
var options = {
    'slug': 'stdlib-js/stdlib/master',
    'file': 'README.md'
};
var err = validate( opts, options );
if ( err ) {
    throw err;
}