redshift: Move period_t to header
redshift: Add hooks for user actions on period switch

Hooks are executable scripts in ~/.config/redshift/hooks/ that
are run when a certain event happens. The first parameter to the
script indicates the event and further parameters may indicate
more details about the event.

The event "period-changed" is indicated when the period changes
("night", "daytime", "transition"). The second parameter is the old
period and the third is the new period. The event is signaled when
Redshift starts up with the old period set to "none".
redshift.1: Add description of hooks in man page
README: Add flattr button at the end
Merge branch 'hooks-dir'