java 8 support and support for passing configuration to Eclipse compiler

Also support for passing compilerArguments like the below. These are prefixed by with a '-' by [AbstractCompilerMojo](https://github.com/apache/maven-plugins/blob/trunk/maven-compiler-plugin/src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java#L564) 
```
 <configuration>
       <compilerId>eclipse</compilerId>
       <source>1.8</source>
       <target>1.8</target>
        <compilerArguments>
                <org.eclipse.jdt.core.compiler.compliance>1.8</org.eclipse.jdt.core.compiler.compliance>
         </compilerArguments>
 </configuration>
```
You might want to check the key actually starts with '-' before trimming it, otherwise you would get odd results if the compiler mojo ever adds a key without a dash. Also if you iterate over the entry set then you can avoid the extra step to lookup the value, as each entry gives you both the key and value.