Add 'subscribe' property to IResolverOptions

[blazarus]

This fixes issue #372. It seems like the 'subscribe' property was simply missing from the interface definition.

I've tested this by yarn linking the change against another project where I was running into the issue and verified I no longer hit the issue with this diff.
npm run test and npm run lint both pass locally.

[apollo-cla]

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[ItsEcholot]

Thanks a lot @blazarus for this fix. Could this be merged as fast as possible and included in the next release?

[stubailo]

Thanks!

[stubailo]

Merged and published in 1.2.1