get_tag_insert() function
Insert a tag (found in an shtml in the form of "" into the file. The tag's name is stored in ssi->tag_name (NULL-terminated), the replacement should be written to hs->tag_insert (up to a length of LWIP_HTTPD_MAX_TAG_INSERT_LEN). The amount of data written is stored to ssi->tag_insert_len. @todo: return tag_insert_len - maybe it can be removed from struct http_state?