Adds a custom exporter option on command line args

[nazulg]

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Fix #1967

This code adds the ability to include a custom exporter from command line arguments.

The user can use the option --customExporter "{assembly-qualified name of exporter}" and it will load the exporter into the config.

Please let me know your thoughts on this approach.

Thank you.

[timcassell]

It doesn't make sense to have the RegisterCustomExporter function if you're just going to create it from the type and ignore the registered name. I like being able to use it from the type, but it should also work from the registered name.

Also, the command line option should support multiple values (like --runtimes). And I would prefer --exporters

[Edit] I see --exporters already exists for built-in exporters. Just expand it to support custom types.

[nazulg]

I updated the PR based on the comments. In this change, a custom exporter can be added to the existing exporters collection.

If the exporter name is not found in the default list, then it tries to create an exporter from the argument as it will be considered to be a type name using the assembly name.

It is still in progress, let me know what do you think of this approach.

[timcassell]

LGTM, unsure if we do want to support registering custom names (to be able to use simplified names as well as full assembly-qualified types). What do you think @adamsitnik?