function decrementHeaders( str, n ) {
	return replace( str, RE_MARKDOWN_HEADER, '$1' + repeat( '#', n ) );
}
Decrements all Markdown headers by a specified number of levels.

@private
@param {string} str - string to modify
@param {number} n - number of levels to decrement
@returns {string} modified string

@example
var str = '## Foo\n\n### Bar\n';
var out = decrementHeaders( str, 1 );
// returns '### Foo\n\n#### Bar\n'

@example
var str = '# Beep Boop\n\n## Baz Bar';
var out = decrementHeaders( str, 2 );
// returns '### Beep Boop\n\n#### Baz Bar'