Source File Filtering

This PR adds general support for file filtering when scanning for schemas and configuration for that feature in the Maven plugin.

Usage:
```
<plugin>
    <groupId>org.jsonschema2pojo</groupId>
    <artifactId>jsonschema2pojo-maven-plugin</artifactId>
    <version>0.4.3-SNAPSHOT</version>
    <configuration>
        <sourceDirectory>${basedir}/schema</sourceDirectory>
        <includes>
            <include>**/*.json</include>
        </includes>
        <excludes>
            <exclude>exclude.json</exclude>
        </excludes>
        ...
    </configuration>
    ...
</plugin>
```
This feature only works with the `sourceDirectory` option and not with `sourcePaths`.