Extend cli tool with app management: ./elefant install <appname>

[jbroadway]

Note: This issue is dependent on issue #182

The command would mirror the GUI app/theme installer on the command line, and would take the name of the app or theme as its argument, getting the installation details from the app/theme directory API.

Usage would be:

$ ./elefant install <app-or-theme-name>

Additionally, there could be a matching update command:

$ ./elefant update <app-or-theme-name>

Or even:

$./elefant update all

To update the core and all apps at the same time.

[jbroadway]

Update:

Apps/themes can be installed via ./elefant install $APP where $APP can be the URL of a zip file, a Github repository, or the shorthand username/repository reference to a Github project.

Updates via ./elefant update $APP or ./elefant update all are not yet implemented.

[jbroadway]

The issue with adding apps to the updater is that we need a unified way to deal with checksums for all apps. A solution would be to create a command for developers to package their apps and publish updates, which could connect to the Github API to create a pull request to https://github.com/elefantcms/checksums with the new checksums for the release, so that the apps can be updated securely just as the core is now.

The command might be:

./elefant release <app-or-theme-name> <version>

Also: We need to make checksums for new installs too.