Add "edit" command

[javiplx]

This change adds an "edit" command, which is basically an ins+set. The main changes respect to the use of both commands in sequence is that the node is not inserted if it already exists (so, "edit" is not usable for labels with multiple entries). And it also adds a "comment" flag which search for a comment with the label on it, and insert the new node after it. This means that

command=edit label=remote_user comment=True ...

will insert the node after

# remote_user = ...

if that line exists, and after path if not. I've used this only on .ini files up to now, but should work in the general case.

[paluh]

@javiplx, please give me a few hours (I hope I will find them this evening), so I can read and test your propositions.

[javiplx]

No hurry from my side, as I'm already using it ...

[jippi]

Any news on this?