Support capsule "modes."

Add ability to define "modes" in the capsule configuration:

```
<modes>
  <mode>
    <name>my_mode</name>
    <manifest>
      <entry>
        <key>Args</key>
        <value>-mymodeswitch - mymodeswitch2</value>
      </entry>
    </manifest>
  </mode>
</modes>
```

```
java -Dcapsule.mode=my_mode target/mytarget-capsule.jar
```