Add formatting options to format configuration

[taalas]

It seems that only raw values from the current version configuration sets can be output using the format string. When outputting times with single digit minutes this leads to timestamps that look like: "8:8" (8 minutes after eight).

It would be great if there were some options for simple variable transformations (like padding a variable with zeroes in this case). e.g.

{$timestamp.hour}:{$timestamp.minute|pl:0:2}

for padding from left to size 2 with zeroes etc.