Add section test TitleNumberedGood0000

Tests basic parsing of titles that begin with a number.
lex.go: Add number comments to itemElement

Helps for debugging.
lex.go: Add isEnumList()
lex.go: Add isArabic()
lex.go: Add lexEnumList()

Very early stages for this function.
parse.go: Remove reflection in Tree.parse()
parse.go: Remove uneeded * operator on Tree.Nodes
node.go: Add EnumListType
node.go: Add EnumAffixType
node.go: Add EnumListNode
parse.go: Add Tree.enumList
Add section test TitleNumberedGood0100

Tests for correct section parsing when the section has a numbered title and the
lines preceeding it are enumerated lists.

To get this test passing required the implementation of basic enumerated list
support. Enumerated lists are not yet officially supported.