Add basic monitors

[GuiltyDolphin]

_More to come (feel free to comment suggestions)_

The following monitors should be provided (at least):

Base

• default - The base-most trigger

First-level monitors

• trigger (default) - used to run every instance's :trigger function when its own is executed.

Second-level monitors

• hook (trigger) - wraps hooks, triggering when they are run.
• tick (trigger) - interval timers, run every X seconds.
• span (trigger) - supports an 'active' period, which may be tested. Can be active (or not) while enabled. Example: an instance might have an active period that is during daylight time.

Third-level monitors

• expression-value (hook) - instances are run when the value of an expression changes (via post-command-hook) <--- the original monitor
• time-span (span) - instances are active between two times, with optional repeats (every day, for example).