String::substr now returns a StringView
use StringView in word_db.cc
use words=all rather than words=buffer for default insert word completion
SelectionList no longer inherit from std::vector
Add client server as a kakoune feature in README
Refactor DynamicRegexHighlighter, search hl uses Search colalias

fixes #122
Fix show_matching highlighter when opening char is the first buffer char

Fixes #119
Fix :delbuf! not forcing buffer deletion

Fixes #116