bin: add --help and --version

Close #220 

/to @seanbrookes please review
/cc @crandmck PTAL if the help text is ok

The help message in verbatim:

```
$ strong-studio --help
Usage
  strong-studio [options]

Options
  --cli   Start the studio backend only, do not open the browser.

The program must be run in the project directory created by
`slc loopback`

StrongLoop Studio will use a different port number each time
you run it. You can provide a specific port number via
the environment variable PORT, for example:
  PORT=4000 strong-studio
```