Add kore_buf_replace_string().

kore_buf_replace_string allows you to replace occurances of a certain
string with something else.

Example:
	char	*username = "Joris";

	page = kore_buf_create(static_len_html_profile);
	kore_buf_append(page, static_html_profile, static_len_html_profile);
	kore_buf_replace_string(page, "%name%", username, strlen(username));