feat(emacs/lean-flycheck): detect and use flycheck error buffer width

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.
feat(emacs/lean-flycheck): detect and use flycheck error buffer width