[REQ] Allow block elements in table fields

[mhatzl]

Currently the syntax only allows inline elements inside table fields.
However, sometimes block elements like lists or tables may want to be placed inside table fields.

Using some fancy markup to allow block elements would most likely make tables unreadable,
so the better option might be to define memorables or macros returning block elements,
and use these memorables/macros in the table field.

Caveats:

• No other content must be allowed inside the fields besides one memorable/macro returning a block element.

  Multiple block elements would need to be separated by blank lines, which is not possible inside a field.
  Therefore, only one memorable/macro returning block elements must be allowed per field.

• Do not consider headings as new sections

  Headings might be added in table fields, but they must not be considered as new document sections.
  This behaviour is the same as for quotation blocks.

• Nestception

  To prevent tables in tables in tables in tables, it might be better to restrict block elements at a certain depth.
  Or just leave it up to the user?

[mhatzl]

About nestception:

As mentioned in issue #37, the allowed list depth will be limited to maximum of 6 nested lists.
This limit could also be used for tables, meaning that it is only allowed to nest tables up to a depth of 6.
A warning should be set at a depth of 4, because this is probably already unreadable.