Allow enabling support for custom imports

[mjrousos]

Currently, try-convert doesn't work with projects that import unrecognized targets and props. This makes sense for a conservative migration approach, but prevents the tool from being useful on a lot of customer projects that import targets files specific to their organization or from reference NuGet packages.

It would be really useful to have an opt-in way of having try-convert work with projects using unrecognized props and targets files. I'm not sure whether the better behavior would be to remove those imports or leave them (I think leaving them or perhaps leaving them commented out would be best), but somehow being able to run try-convert on such projects would be valuable.

[rmaloo]

+1 we too are having similar issue using try-convert tool

[mjrousos]

I'm working on a fix for this. Should have a PR early next week.