Detect the width of flycheck error list buffer and set pp.width when
calling flycheck. It uses the same heuristic that `split-window-sensibly`
function is using.
A user can also set `lean-flycheck-msg-width` custom variable to enforce
a certain width.
Close#205.
The key idea is to pass a continuation function when we call
lean-server-send-cmd function. The passed continuation function is
called by lean-server-event-handler. Until the buffer is ready, the
event handler will be called in interval (lean-server-retry-time, 0.1
sec by default). When we have 'NAY' for INFO command, it will re-send
INFO commands to lean-server until we have one without 'NAY'.
Close#123
