Login & logout commands


Uses the new functionality added to [go-netrc](https://github.com/bgentry/go-netrc) to both read and write .netrc files while also preserving whitespace in the original file.

```
$ hk help login
Usage: hk login <email>

Log in with your Heroku credentials. Input is accepted by typing
on the terminal. On unix machines, you can also pipe a password
on standard input.

Example:

    $ hk login user@test.com
    Enter password: 
    Login successful.
```

```
$ hk help logout
Usage: hk logout

Log out of your Heroku account and remove credentials from
this machine.

Example:

    $ hk logout
    Logged out.
```