Add CLI to Yeoman template

[spoenemann]

See discussion in #144. We should include a CLI in the generator-langium template so users have a clear path to implement a code generator.

I propose to add two files:

• src/cli/<language-id>-cli.ts is the entry point of the CLI and uses commander to provide commands and options.
• src/cli/generate.ts contains the implementation of a generate command. The initial "hello world" example could generate JavaScript code that prints Hello {name}! for every greeting in a source file.

See also #131.