Better 'describe' command output, supersedes #134.

Example:

```
$ bin/riak config describe bitcask.expiry
Documentation for bitcask.expiry
By default, Bitcask keeps all of your data around. If your
data has limited time-value, or if for space reasons you need to
purge data, you can set the `expiry` option. If you needed to
purge data automatically after 1 day, set the value to `1d`.
Default is: `off` which disables automatic expiration

   Datatype     : [{atom,off},{duration,s}]
   Default Value: off
   Set Value    : undefined
   app.config   : bitcask.expiry_secs 
```

Also, sometimes the default value will be wrapped in quotes if it's a string. We need a better name for "app.config" too.

This pull-request's output looks more like this:

```
Documentation for bitcask.expiry
By default, Bitcask keeps all of your data around. If your
data has limited time-value, or if for space reasons you need to
purge data, you can set the `expiry` option. If you needed to
purge data automatically after 1 day, set the value to `1d`.
Default is: `off` which disables automatic expiration

   Valid Values: 
     - the text "off"
     - a time duration with units, e.g. '10s' for 10 seconds

   Default Value: off
   Set Value    : undefined
   Internal key : bitcask.expiry_secs
```