Add `conda launch` command

    $ conda launch -h
    usage: conda launch [-h] [COMMAND]
    
    Launches an application installed with Conda.
    
    positional arguments:
    COMMAND     package to launch
    
    optional arguments:
    -h, --help  show this help message and exit

    $ conda launch ipython-qtconsole  
    Fetching package metadata: ...
    Started app. Some apps may take a while to finish loading.

    $ conda launch ipython-qtconsole --json
    {
      "fn": "ipython-qtconsole-2.1.0-py34_0.tar.bz2"
    }

    $ conda launch conda                   
    Fetching package metadata: ...
    Error: Not an application: conda-3.5.5-py34_0.tar.bz2

    $ conda launch conda --json
    {
      "error": "Not an application: conda-3.5.5-py34_0.tar.bz2"
    }

    $ conda launch nonexistent_package       
    Fetching package metadata: ...
    Error: App nonexistent_package not installed.

    $ conda launch nonexistent_package --json
    {
      "error": "App nonexistent_package not installed."
    }

    $ conda launch ipython-qtconsole-2.1.0-py34_0.tar.bz2
    Started app. Some apps may take a while to finish loading.

    $ conda launch ipython-qtconsole-2.1.0-py34_0.tar.bz2 --json
    {
      "fn": "ipython-qtconsole-2.1.0-py34_0.tar.bz2"
    }
    
TODO:

- `conda info package1 package2 ... --json`
- `conda list --json`
- `conda info ipython-qtconsole-2.1.0-py34_0.tar.bz2 [--json]`?(retrieve metadata such as install status, features,...)
- `conda search --json`, etc.