type-break-mode: Make variable `nil' by default.

type-break-mode: Make variable `nil' by default.
type-break-mode (function): If setting to t and mode was already enabled,
don't reschedule breaks or reset keystroke counter.

type-break-good-rest-interval: New variable.
type-break-time-difference: New inline function (defsubst).
tyype-break-time-last-break, type-break-time-next-break,
type-break-time-last-command: New variables.

type-break-check: Reset timers and counters if user has been idle more than
type-break-good-rest-interval seconds (assuming it's set).
Never set keystroke counter to be less than the min threshold.

type-break: Admonish user if s/he rested less than
type-break-good-rest-interval seconds (assuming it's set).

type-break-demo-hanoi: Eat a char when quitting.

type-break-statistics, type-break-guestimate-keystroke-threshold: New
commands.