* plugins/html/html_read.c (html_read_table): Pass in GnmHtmlTableCtxt
from outside. When page contains multiple tables, this lets us
concatenate them instead of creating multiple sheets.
(html_search_for_tables): Add GnmHtmlTableCtxt parameter. Don't call
the function recusively when this node is a table. This means that
tables within the table aren't treated as independent tables.
(html_file_open): Add a GnmHtmlTableCtxt local variable and pass it as
an argument to html_search_for_tables.