Attribute lists are represented as associative lists in Lua. Pure
associative lists are awkward to work with. A metatable is attached to
attribute lists, allowing to access and use the associative list as if
the attributes were stored in as normal key-value pair in table.
Note that this changes the way `pairs` works on attribute lists. Instead
of producing integer keys and two-element tables, the resulting iterator
function now returns the key and value of those pairs. Use `ipairs` to
get the old behavior.
Warning: the new iteration mechanism only works if pandoc has been
compiled with Lua 5.2 or later (current default: 5.3).
The `pandoc.Attr` function is altered to allow passing attributes as
key-values in a normal table. This is more convenient than having to
construct the associative list which is used internally.
Closes#4071
