Use tag parser for 'blockScript', 'persistendBlockScript' and 'qbar pipe'.
'blockScript' no longer handles (and allows) exit codes other than 0.

This reverses the control flow between 'Cli' and 'Server'

This removes Pango support for script blocks. Persistent scripts don't
have an alternative way to format their output for now.

This reverts commit 66778fdb.

It is now much easier to write a custom Main.hs that generates a custom
bar configuration or that changes the argument parser.