msgthr 1.2.0 - Msgthr#add callback support

This release adds callback support to the Msgthr#add method,
allowing callers to track progress and potentially group
message.  Thanks to Dimid Duchovny for this feature.
Discussion about it begins here:

  https://80x24.org/msgthr-public/CANKvuDf7esPfy3eQ0B8aQjg4sTYTcxR_LNNWeDBcENFwmyC_3g@mail.gmail.com/t/

4 changes from Dimid Duchovny:
      add callback to Msgthr#add
      test: add a more complex test for add_child callback
      test: fix add_child callback test
      doc: document block parameter of Msgthr#add