add plugin overriding feature

[prevostcorentin]

PR related to issue #1201.

[prevostcorentin]

This PR can be improved:

If two plug-ins defines override=true, the second one in the read order will replace the first one. Because the plug-in specification table is filled asynchronously, two overrides will bring hazardous situations.

Two solutions comes in mind:

• Explicitly forbid more than one override for a given plug-in
• Explicitly specifiy an override priority

I don't see any situation where two overrides would be necessary. This feature should be used when you are developing a feature you need on a plugin you pull from github and the feature is not yet merged. While waiting for the feature to be merged or not, you want to keep a configuration as clean possible (as I like to do).

I would personally disabling the ability to define more than one override for a given plug-in.

Best regards.

[prevostcorentin]

Many undesirable changes are brought by an auto format feature.
It can be removed if needed.