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.
@baev addExtension(String), addExtensions(String...) - :+1:
Cc @vania-pooh 

Regarding splitting extension configuration by fields - good idea, but it is too verbose configuration in Maven, Jenkins, CLI for so advanced and non regularly used feature. Please consider additional two fields: dependency type (which probably is only jar for UrlClassLoader) and classifier. Also if keep addExtension(String) method we have to concatenate artifact details back to String.

Looking at "canonical" String format, it is compact, already parsed by aether, and I would say widely recognized (e.g. used in Gradle build tool).

May be we could agree on mixed option:
```xml
<reporting>
    <plugins>
        <plugin>
             <groupId>ru.yandex.qatools.allure</groupId>
             <artifactId>allure-maven-plugin</artifactId>
             <version>{latest.version}</version>
             <configuration>
                 <extensions>
                     <extension>groupId:artifactId:version<extension>
                     <extension>groupId:artifactId:jar:jdk5:version<extension>
                 <extensions>    
             </configuration>
        </plugin>
    </plugins>
</reporting>
```
What do you think?