Issue2394 multiple markdown exporters not possible even with different names

[bstordrup]

Changed merging of exporters to check for duplicates by exporter.Name instead of the System.Type.

Added a test to verify that two MarkdownExporters with different named (GitHub and Atlassian) are accepted and both present in the final list (count is 2).

The original that removes duplicates still passes.

Fixes #2394

[timcassell]

Maybe it would be better to sub-class the MarkdownExporter rather than changing the logic. But I don't feel strongly about it either way.

[bstordrup]

Maybe it would be better to sub-class the MarkdownExporter rather than changing the logic. But I don't feel strongly about it either way.

That could be an option too, but I think it would complicate the class hierarchy. The different variations of MarkdownExporter are only differing in various values of the MarkdownExporter properties. So it would lead to specific classes not changing functionality but just setting properties. With that in mind, I would prefer the current way with declaring them as static properties on MarkdownExporter.

Also, there is no public constructor on MarkdownExporter class, which also complicates creating specific classes and instantiating these.

[adamsitnik]

LGTM, thank you for providing the tests @bstordrup !