Table markup

(this depends on pull request #21)

Allow lists, code, nowiki, and plain text inside table cells. Table cells cannot contain single-item lists, to preserve backwards compatibility.

Example:

    |* Foo
    * Bar
    ** Baz|world|{{{look
    some
    code}}}|
    |=And I didn't|=break|=tables|

Plain text still cannot contain linebreaks, nor can blocks be followed by trailing linebreaks---that still breaks the table. However, linebreaks can exist where allowed by the containing element: to start new list items, to embed content within list items, inside code/nowiki, following a `\\`, etc.

This goes part way towards fixing REVIKI-341.