Add command line option for GHC version and release.

The only two commands that need them are `add` and `pkgbuild`.

Closes #9

Signed-off-by: Magnus Therning <magnus@therning.org>
Change help texts to start with upper case.

This way they match the help message built into optparse-applicative.

Signed-off-by: Magnus Therning <magnus@therning.org>