Add --version CLI option

[jacg]

• where can more precise GHC version information be found?
• where is the single source of wisdom on ghcide version?

Closes #103.

[ndmitchell]

See https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/options-phases.html 4.12.3 for the things GHC gives you to get the version. Unfortunately it doesn't have enough info. However, there is a base version which forms a 1:1 mapping with GHC version. https://wiki.haskell.org/Base_package#Versions has a partial map, but someone seems to have forgotten to define it recently.

I suggest we add System.Info.Extra.compilerVersionFull :: Version which gives us what we need. We then propose that for addition to GHC at some point. We would build it using base versions. It's a pain since it needs updating on every point release.

[jacg]

So should we just merge this as is, to provide --version, and leave the more precise GHC version info for a separate PR?

[ndmitchell]

Makes sense, yep.

[ndmitchell]

I looked into adding precise GHC version in extra, but I only have the base version available, and that's insufficient as there are multiple GHC versions with the same base.

However, Cabal predefines:

#define VERSION_ghc "8.6.5"

So just using CPP should be sufficient.

[cocreature]

Nice work, thank you!