Add support for QQmlExtensionPlugin-based plugins

[dragly]

Add vendor.h with qpm::init(app, engine) that initializes each plugin listed in qpm.json.
Each plugin must be listed with the name of the C++ class and the uri this class expects in registerTypes. Each header needed for these classes must also be listed in qpm.json:

    "headers": [
      "src/mymoduleplugin.h"
    ],
    "plugins": [
      {"class": "MyModulePlugin", "uri": "MyModule"}
    ]

In main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <vendor.h>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    qpm::init(app, engine);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

For readability in the pull request, this commit has excessive whitespace in the vendor.h template. This should be fixed later.

The implementation is similar to the proposal mentioned here:
https://trello.com/c/jvIlnOAM/8-add-some-convenience-code-for-registering-modules

PS: This is my first attempt at coding in Go. Please excuse any lack of proper style :)