Add 'a' alias to angle brackets text object

fixes #39
Rework regions highlighter, refactor and extract line tracking
WordDB now uses a LineChangeWatcher based implementation
cpp.kak: re-enable region highliting for comments and strings
update gdbinit
fix clang auto completion at eol
Rework command completion to allow partial token completion

Implement hook completion.

fixes #44
Honor incsearch option for select/split/keep

fixes #37
Remove session socket on error signal
remove unused Singleton::delete_instance