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.