Added support for explicitly loading IGraphicsProcessors

Auto-loading IGraphicsProcessor means that all DLLs are loaded. This can cause issues if you have reference issues in your bin (don't ask!)

This change is off by default and controlled via config. If autoLoadPlugins is false then only the plugins defined will be loaded.
```
<processing>
  <plugins **autoLoadPlugins="false"**>
    <plugin name="Format" **type="ImageProcessor.Processors.Format, ImageProcessor"**/>
    <plugin name="GaussianBlur" type="ImageProcessor.Processors.GaussianBlur, ImageProcessor">
      <settings>
        <setting key="MaxSize" value="22"/>
        <setting key="MaxSigma" value="5.1"/>
        <setting key="MaxThreshold" value="100"/>
      </settings>
    </plugin>
  </plugins>
</processing>
```