Add ToEnd constant to use as special end index in Slice

Merge pull request #234 from PuerkitoBio/wip-slice-end

Add ToEnd constant to use as special end index in Slice, closes #232.

Add `AddBack*`, deprecate `AndSelf`

See #229 .

Add SetHtml and SetText

v1.1.0

Merge branch 'glebtv-setHtml'

Optimize allocations for Selection.Text

v1.0.2

Update README.md

Optimize for large documents

• significant performance boost when dealing with large documents - related issue: #126