DisplayBuffer: optimize Text and ReplacedBufferRange as well
Move selection update code to SelectionList
add SelectionList::check_invariant
Editor: undo and redo select all ranges modified instead of the last one
Editor: tweak behaviour on undo/redo when selecting modified ranges
BufferCompleter: avoid modifying the buffer when not needed
add a completers option for specifying which completer to use for BufferCompleter
add specialized contains function for unordered_set
add unordered_set option support, use it for completers
Option can be declared with a checker function.

The checker function may throw if to be set option value is invalid.
Implemented with the completers option.
remove GOALS file
support specifying colors with RGB components
Use more std::* for string handling
fix regex prompt whith empty regex
fix overlaps(Range, Range)
tweak Range::merge_with behaviour
Editor::select(Selection, SelectMode::Extend) now only keeps the extended main selection
colalias can reference another alias
Full object selection support for paragraphs and sentences
add doc/keymap