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.
+1 for @baev