Because it doesn't make sense to replace the date placeholder at
process creation, it has to be replaced at every start of the process.
On process creation only the static placeholders (such as process ID)
are replaced. Dynamic placeholders (so far only "date") are not
replaced. On process start, a callback has been introduced that gives
the chance to change the command line. This is the point where
the restreamer replaces the date placeholders.
1. Allow variables in placeholders for parameter values, e.g.
{rtmp,name=$processid}. The variable starts with a $ letter.
The recognized variables are provided with the Replace func.
2. The template func recieves the process config and the name of
the section where this placeholder is located, i.e. "global",
"input", or "output".
