Add `--version` flag to CLI

[dhirschfeld]

Having a --version flag is pretty standard for a CLI as it lets the user know what version of the tool they're using, and hence also helps with bug reports.

> sqlcmd --version
sqlcmd.exe: error: unknown flag --version

[dhirschfeld]

Am I missing something obvious and there's another way to know what version of sqlcmd I'm running?

[stuartpa]

Fixed in #100