Introduce rs_color for reading color resources

It handles reading *-fg and *-bg resource as well as default
values.
Fix indent
Pass $now to tab_activity_mark to avoid calling urxvt::NOW all the time

µoptimisation.  Probably does not make any difference, but somehow
makes sense to me. ;)
Allow overwriting of the initial activity mark character

-1 was not accepted as a valid timeout anad besides it makes more
sense for 0 being the timeout for the initial activity mark.  Changing
comparison from > to >= moves all marks by one second (which somehow
alters the behaviour of the resource but it should not be too visible
to users) and lets 0 match initial output.
Add support for matching activity characters
Introduce bell handling
Rename lastActivity to last_activity to be consistant in naming
Introduce rs_bool function and rename my_resource to rs_text