feat: run cmd accept custom manifest parser

[chrmod]

following up #1019 (comment)

[coveralls]

Coverage remained the same at 100.0% when pulling f26cc03 on chrmod:run-custom-manifest-parser into 6dde038 on mozilla:master.

[kumar303]

Thanks, this seems reasonable. We can't guarantee that it won't break in the future in case we add more manifest parsing but, hey, it's worth a shot.

I think Appveyor is drunk so I'm just going to merge it.

[chrmod]

awesome, thanks. two more to go

[caitmuenster]

Thanks so much @chrmod! Your contribution has been added to our recognition wiki and your Mozillians profile will be vouched. :)

Welcome onboard!

[chrmod]

@caitmuenster that is great :) thank you