Add --version argument

[ivan-aksamentov]

Currently there does not seem to be a way to query a version of the CLI, so users are completely in the dark about what they are using:

$ docker run -it --rm neherlab/pangraph:latest pangraph --version        
pangraph --version: unknown command
Run 'pangraph help' for usage.

$ docker run -it --rm neherlab/pangraph:latest pangraph version
pangraph version: unknown command
Run 'pangraph help' for usage.

$  docker run -it --rm neherlab/pangraph:latest pangraph help   
usage: pangraph <command> [arguments]

    a tool for aligning large sets of closely related genomes in the presence of horizontal gene transfer

The available commands are:

  build       align genomes into a multiple sequence alignment graph
  generate    outputs a simulated sequence alignment graph
  help        prints usage information for a given command
  polish      realigns pancontigs of multiple sequence alignment graph
  marginalize computes all pairwise marginalizations of a multiple sequence alignment graph
  export      exports a pangraph to a chosen file format(s)

The expected arguments are:

  passed directly to the chosen command

Would be nice to add a --version flag or a version command.

Extra points for synchronizing the version reported with the version in the manifest.