Support for Marginalia multidoc generation

Hi,

I'd like to use multi-doc generation with Marginalia and changed the plugin to support this.

I created an integration test as well, but for some reason my machine has problems executing any integration test. Please bear with me.


From the commit message:

Marginalia can generate documentation for each namespace as a separate file. This commit adds a configuration option to the "marginalia" map to support this feature.

Usage:

```xml
  <plugin>
    <groupId>com.theoryinpractise</groupId>
    <artifactId>clojure-maven-plugin</artifactId>
    <version>1.3.14-SNAPSHOT</version>
    <configuration>
      <marginalia>
        <multi>true</multi>
      </marginalia>
    </configuration>
  </plugin>
```