Plugin Settings : define content negociation based on URI

[sgrellet]

We will need to be able to configure in the Plugin settings what format (http Accept) should be used with which URI.

I know it kinds of mimics what GMLAS conf does (see gmlasconf-inspire.xml). But that's also needed in XML mode. Sure we can manage to keep this consistent (see #38 ) The plugin and GMLAS conf could come together with pre-configured settings for well know repo (ex: INSPIRE, ...)