Add useSubscription hook

[stramel]

Closes #70

This was mostly ported from the official repo.

I wasn't sure what all you wanted to display in the docs.

@morrys @renanmav @sibelius @jstejada @rbalicki2 PTAL

[morrys]

hi @stramel,
for documentation it is also necessary to add the "use-subscription" entry here: https://github.com/relay-tools/relay-hooks/blob/master/website/sidebars.json#L6

[morrys]

@stramel, thank you for your contribution :)
i hope to release a new version with this hook together with the PR #71 on the weekend :)