Allow to change merge message

[Itori]

• Add the possibility to change the merge message
• Add the possibility to automatically add the one line description from merged commit

I also would like to be able to save those settings, but i don't really know where :

• Add log : should be saved, in EffectiveSettings ?
• Nb log : the config setting merge.log can be used to change this value, but i don't know how to access it
• Add merge message : i don't think this option should be saved