action-get in jujuc

Hooks can now run the command action-get to retrieve the map of params
passed with the action hook.  In the case of a nil params map, an empty
value will be passed.  Format for the command is as follows:

`action-get [--output=<json|yaml>] [key[.inner.value...]]`

For example,

`action-get --output=json outfile.compression` might output
```json
{"quality":"high","type":{"utility":"bzip2","flags": "--small"}}
```


In the case of a nonexistent key, a nil interface will be returned to
the command Stdout writer, which will display it as an empty string or
null in the case of JSON output.
drop the else here, it indents everything else unnecessarily, given the above branch returns early.