Add support for merge-replace option

[LukeTowers]

It would be great if we could disable the merging of the replace property as it causes issues when the composer.json file being merged from is included by a package that is itself loaded via the main composer.json file. See wintercms/wn-user-plugin#12. Merging these properties were added in #60 but there was no way to disable that functionality.

Failing that it would be good if there was some way to indicate not to merge the configs of any packages that are already included by the main composer.json file since their configs will already be evaluated by Composer when they're required in the main composer.json file.

[eileenmcnaughton]

This is what I'm looking for too - not sure if using self:version makes a difference - I'm trying it but it's giving me other issues