Minor mode for on-the-fly syntax checking.
When called interactively, toggle `flycheck-mode'. With prefix
ARG, enable `flycheck-mode' if ARG is positive, otherwise disable
it.
When called from Lisp, enable `flycheck-mode' if ARG is omitted,
nil or positive. If ARG is `toggle', toggle `flycheck-mode'.
Otherwise behave as if called interactively.
In `flycheck-mode' the buffer is automatically syntax-checked
using the first suitable syntax checker from `flycheck-checkers'.
Use `flycheck-select-checker' to select a checker for the current
buffer manually.
Uses keymap `flycheck-mode-map', which is not currently defined.
(fn &optional ARG)