Add support for vendor extensions in the 'info' section

[cjelger]

Like in the previous version of the Swagger at plugin https://github.com/kongchen/swagger-maven-plugin, it would be nice to support vendor extensions in the 'info' section of the plugin.

I can take care of that and open a PR, but I'm waiting for some official approval from my employer to be able to work on that project.

[langecode]

Solved by PR #14