#46 Spell checking

[Laurens Holst]

Spell checking functionality when previewing.

Could be implemented as a post processor which is only
applied on previews.

Question: how to determine the language. This is a
broader topic, because we want to add the proper
xml:lang attributes as well. We should try our best to
auto-detect this? Maybe we can base it on some fuzzy
logic involving default language settings + character
frequency?

~Grauw