Adds --numeric-version and --ghc-numeric-version

[angerman]

This allows to parse the ghc version from doctest with any parser
that can parse ghc --version already.

Fixes #157

[angerman]

I'm not sure if we really want this. As doctest --version already provides this information. See

$ doctest --version
doctest version 0.11.2
using version 8.3.20170502 of the GHC API
using /Users/angerman/mobile-ghc/bin/ghc-8.3.20170502

This though would provide it in the identical format the ghc --version provides it:

$ doctest --ghc-version
The Glorious Glasgow Haskell Compilation System, version 8.3.20170502

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.3.20170502

[phadej]

I'd output just a 8.3.20170502 so the output can be used by other tooling.

EDIT compare how stack has stack --hpack-numeric-version; maybe the flag should be --ghc-numeric-version ?

EDIT and should there be bare --numeric-version which would output 0.11.2 doctest version string

[angerman]

Alright that's more sensible (to me at least):

$ doctest --ghc-numeric-version
8.3.20170502

$ doctest --numeric-version
0.11.2

[phadej]

👍

[sol]

I'm closing this in favor of #159 (I prefer --info, analogous to ghc --info).