cli: Add mongodb to help message.

[jchappell82]

While installing a test cluster tonight to try out the mongodb component, I noticed that the mongodb module is not mentioned by flynn --help. This tiny PR adds that. I looked through the CLI tests and didn't see anything explicitly testing the output of --help, but if I missed that, I'd be happy to update it.

vagrant@flynn:~/go/src/github.com/flynn/flynn$ flynn --help
usage: flynn [-a <app>] [-c <cluster>] <command> [<args>...]

Options:
    -a <app>
    -c <cluster>
    -h, --help

Commands:
    help        show usage for a specific command
    install     install flynn
    cluster     manage clusters
    create      create an app
    delete      delete an app
    apps        list apps
    info        show app information
    ps          list jobs
    kill        kill jobs
    log         get app log
    scale       change formation
    run         run a job
    env         manage env variables
    limit       manage resource limits
    meta        manage app metadata
    route       manage routes
    pg          manage postgres database
    mysql       manage mysql database
    mongodb     manage mongodb database
    redis       manage redis database
    provider    manage resource providers
    docker      deploy Docker images to a Flynn cluster
    remote      manage git remotes
    resource    provision a new resource
    release     manage app releases
    deployment  list deployments
    export      export app data
    import      create app from exported data
    version     show flynn version

[josephglanville]

Thanks!