Add init command to the CLI

This commit adds a new command to the CLI that allows to initialize a new
project from the command line. It uses the Spring initializr service to
actually generate the project.

The command offers two main operations:

1. Listing the capabilities of the service (--list or -l). This basically
dumps the defaults of a given service and the list of dependencies and
project types it supports
2. Generating a project. By default, http://start.spring.io is used and
its configured defaults are applied. Running spring init would therefore
have the same effect as clicking the 'generate project' on the UI without
entering any extra information. No file is overwritten by default.

The generation can be customized with the following options:

* --boot-version (-bv) Spring Boot version the project should use
* --dependencies (-d) comma separated list of dependencies to add to the
generated project
* --java-version (-jv) Java version to use
* --packaging (-p) the packaging for the project (jar, war)
* --target the url of the service to use

The actual type of the project can be defined in several ways:

1. Using the --type (-t) option that identifies a type that is supported
by the service
2. A combination of --build and/or --format that can be used to uniquely
identify matching these tags. Build represents the build system to use
(e.g. maven or gradle) while --format defines the format of the generated
project.

The project is saved on disk with the name provided by the server through
the Content-Disposition header, if any. It is possible to force it with
the --output option. It is possible to overwrite existing files by adding
the --force (-f) flag.

The --extract (-x) option allows to extract the project instead of saving
the zip archive. By default, the project is extracted in the current
working directory but it is possible to specify an alternate directory
using the --output option.

Fixes gh-1751