feat: add flag to display version info

[macintoshpie]

Context

addresses #16

Changes

• change main.go to allow displaying version/build information. goreleaser will automatically update these variables using ldflags when building the release assets in the future. If a user wants to include this when building themselves (not recommended), they can run the following (with the shell variables set or replaced)

  go build -o modelicafmt -ldflags="-X main.version=$version -X main.commit=$commit -X main.date=$date -X main.builtBy=$builtBy

Example output

╰─$ ./dist/modelica-fmt_darwin_amd64/modelicafmt -v       
modelicafmt v0.1 (SHA 09181bfc4d7482d4dae703af7872e9864fc545d3)
Built 2020-10-06T15:56:56Z by goreleaser