A table within a table should be convertet into a table within table
```
% pandoc -f html -t asciidoc
NestedTables
^D
[width="100%",cols="50%,50%",]
|===
a|
[cols=",",]
!===
!a1 !a2
!===
|b
|c |d
|===
```
A table within a table within a table cannot be converted because asciidoc only
supports two levels of tables.
The table on level 3 is thus converted to level 2 and a warning is produced
```
% pandoc -f html -t asciidoc --verbose
NestedTables
^D
[INFO] Not rendering Table ("",[],[]) (Caption Nothing []) [(AlignDefault,ColWidth 0.5),(AlignDefault,ColWidth 0.5)] (TableHead ("",[],[]) []) [TableBody ("",[],[]) (RowHeadColumns 0) [] [Row ("",[],[]) [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "a1"]],Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1) [Table ("",[],[]) (Caption Nothing []) [(AlignDefault,ColWidthDefault),(AlignDefault,ColWidthDefault)] (TableHead ("",[],[]) []) [TableBody ("",[],[]) (RowHeadColumns 0) [] [Row ("",[],[]) [Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "1"]],Cell ("",[],[]) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "2"]]]]] (TableFoot ("",[],[]) [])]]]] (TableFoot ("",[],[]) [])
[width="100%",cols="50%,50%",]
|===
a|
[width="100%",cols="50%,50%",]
!===
!a1 !
!===
|b
|c |d
|===
```