support plugin configuration for multiple plugins

[NicolasRannou]

in the case where:

• Plugin A requires a user configuration 'thiselement'.
• Plugin B requires a user configuration 'thiselement'.

We have to define this element 2 times in the configuration file.

[PluginA] thiselement=value

[PluginB] thiselement=value

Should we then define it as a general element to avoid re-declaring it?

[General] thiselement=value