Add support for AsciiDoc formatting inside a table cell

[fbolton]

Need to treat header cells and ordinary cells differently, because we need to use a special syntax to enable AsciiDoc formatting inside an ordinary cell. To use AsciiDoc formatting, you need to delimit the cell by starting with a| and ending with a newline, \n. In other words, instead of formating a table like this:

|===
|Header 1|Header 2|

|Foo|Bar
|===

It should be formatted like this:

|===
|Header 1|Header 2|

a|Foo
a|Bar

|===

And then it becomes possible to put fancy formatting inside the table cell (lists, code listings, etc.)