New feature: Use third-party extensions declaratively

[Yasuharu Nakano]

There is a config.groovy having:

extensions {
    url "https://github.com/kobo/gaiden-column-block"
    file "/tmp/my-trial-extension" // OR:  url "file:///tmp/my-trial-extension"
}

A gaiden build would automatically clone a git repository from the url into $HOME/.gaiden/extensions.

A file is not important because it could be substituted by url. It just checks the directory and added the path to extensionDirectories. In case of url, the directory would be copied into ./.gaiden/extensions

A refresh-extensions command might be better like Grails.