Supporting allure extensions (https://github.com/allure-framework/allure...

Issue https://github.com/allure-framework/allure-core/issues/414.

Allure report builder can be configured with extensions (extensions can contain additional SPI implementations, e.g. DataProvider).

Extensions string should be ```<dependency>[;<dependency>]*``` where ```<dependency>``` is string in aether format (```<groupId>:<artifactId>[:<extension>[:<classifier>]]:<version>```).

Later allure maven plugin can be made configurable, as well as Jenkins plugin. For CLI we could configure allure-report-builder via system property or additional argument.
Maybe `addExtensions`? Also I would like to see methods `addExtension(String)` and `addExtensions(String...)`

An example in maven plugin we will set extensions something like that

```xml
<reporting>
    <plugins>
        <plugin>
             <groupId>ru.yandex.qatools.allure</groupId>
             <artifactId>allure-maven-plugin</artifactId>
             <version>{latest.version}</version>
             <configuration>
                 <extensions>
                     <extension>
                         <groupId>...</groupId>
                         <artifactId>...</artifactId>
                         ...
                     <extension>
                 <extensions>    
             </configuration>
        </plugin>
    </plugins>
</reporting>
```

And we get `List<Dependency>`. I wouldn't like convert it to your extension string format