Add --json flag for dhall {decode,encode}

[Gabriella439]

Fixes #714

This adds a --json flag that dhall decode and dhall encode can use to
read/write the equivalent JSON representation of the CBOR. This comes in
handy for the parsing compliance tests which use this CBOR-as-JSON as the
standard representation of the abstract syntax tree.

[Gabriella439]

@f-f: Ah, CI caught a problem with this. cborg-json requires a version of base newer than what GHC 7.10.3 supports (cc: @jneira)

[jneira]

Mmm, cborg-json-0.2.10 uses base (>=4.7 && <4.13) and ghc-7.10.3, base = 4.8.2.0 so, in theory, they could be compatible...

[Gabriella439]

Oh, I completely misunderstood what was going on. It's actually the exact opposite problem: ghc-8.6 uses a version of base that is too new for cborg-json. Let me see if I can get cborg-json to bump its upper bound on base

Edit: Looks like a newer version of cborg-json already did. Ignore this

[Gabriella439]

Looking more closely, cborg-json is not on Stackage so I need to see if the maintainers would be okay with adding it to Stackage

[Gabriella439]

Here is the request to add cborg-json to Stackage: well-typed/cborg#191

[Gabriella439]

Alright, cborg-json was added to Stackage nightly and I fixed dhall to use the latest version which has the expanded bounds on base so this is good to merge now