Add special text substitutions

[omarmir]

::now - the current datetime as string
::second - @now second number (0-59)
::minute - @now minute number (0-59)
::hour - @now hour number (0-23)
::weekday - @now weekday number (0-6)
::day - @now day number
::month - @now month number
::year - @now year number
::yesterday - the yesterday datetime relative to @now as string
::tomorrow - the tomorrow datetime relative to @now as string