add cabal-helper to Stackage Nightly

[juhp]

It would be nice to add the package to Stackage.

With the current release, for Nightly I get:

cabal-plan-0.5.0.0 is out of bounds for:

• [ ] cabal-helper-0.8.1.2 (>=0.3.0.0 && < 0.5). @DanielG. Used by: library, executable, test-suite

ghc-8.6.5 is out of bounds for:

• [ ] cabal-helper-0.8.1.2 (>=7.8 && < 8.5). @DanielG. Used by: test-suite

pretty-show-1.9.5 (Grandfathered dependencies) is out of bounds for:

• [ ] cabal-helper-0.8.1.2 (>=1.8.1 && < 1.9). @DanielG. Used by: executable, test-suite

semigroupoids-5.3.2 (Edward Kmett [email protected] @ekmett) is out of bounds for:

• [ ] cabal-helper-0.8.1.2 (==5.2.*). @DanielG. Used by: library

template-haskell-2.14.0.0 is out of bounds for:

• [ ] cabal-helper-0.8.1.2 (>=2.7.0.0 && < 2.14). @DanielG. Used by: executable, test-suite

temporary-1.3 (Grandfathered dependencies) is out of bounds for:

• [ ] cabal-helper-0.8.1.2 (>=1.2.1 && < 1.3). @DanielG. Used by: executable, test-suite

[juhp]

Sorry this is probably mostly a duplicate of #62

[DanielG]

Like I said in the other issue, happy to take PRs for 0.8 but I'm concentrating on the 1.0 release.