Add option to generate config from current checkout: repoman config --fromcwd

Added the config option `--fromcwd` to generate .repoman.json from the checked out repos in the current working directory:
```
repoman config --fromcwd
```

I find this useful to sync working directories which may be of mixed origin or a subset of those available from my GitHub projects. This also provides an easy way to get started with repoman.

Re-running `config --fromcwd` will regenerate the config (rather than complaining one already exists).