Add a `--version` flag

[jvoisin]

There is no way currently to see what version of harper-ls one is running:

jvoisin@chernabog 13:44 ~ harper-ls --help
Start a language server to provide grammar checking inside of developer environments.

Will listen on 127.0.0.1:4000 by default.

Usage: harper-ls [OPTIONS]

Options:
  -s, --stdio
          Set to listen on standard input / output rather than TCP

  -h, --help
          Print help (see a summary with '-h')
jvoisin@chernabog 13:44 ~ 

It would be nice to provide a --version flag to make it easier to report issues.