Added method for peeking the next line in a text-based decoder
Bugfix: appendByMove implementation was incorrect (called copy constructor)
Use pointers to RawAccount objects in RawConversation

This is because the accounts will be referenced by RawSpeaker objects in
the future. Note that this means RawConversation is no longer trivially
copyable. A move constructor has been added to RawConversation, and it
is intended that RawConversation be move-only.
Added some helper functions for handling accounts in RawConversation
Added design note about decoders
Added support for RawSpeaker data structures in RawConversation

These are needed by the RawMessage structure, itself needed to represent
parsed messages.