Add version flag

For convenience in determining which version of Buffet is installed, add
a `--version` flag.

Change-Id: I1aac363acf2bc4518934038038f0f92c843e4b77
Reviewed-on: https://gerrit.causes.com/27555
Tested-by: Shane da Silva <shane@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>