Add a --java-package-prefix option to flatc

[chuckatkins]

This adds an option to flatc for a prefix to the generated package name in java.

Example:

Running

./flatc -j samples/monster.fbs

generates the following:

MyGame/Sample/Color.java
MyGame/Sample/Equipment.java
MyGame/Sample/Vec3.java
MyGame/Sample/Monster.java
MyGame/Sample/Weapon.java

each containing

package MyGame.Sample;

When using the new --java-package-prefix option, running

./flatc -j --java-package-prefix com.monsters samples/monster.fbs

generates the following:

com/monsters/MyGame/Sample/Color.java
com/monsters/MyGame/Sample/Equipment.java
com/monsters/MyGame/Sample/Vec3.java
com/monsters/MyGame/Sample/Monster.java
com/monsters/MyGame/Sample/Weapon.java

each containing

package com.monsters.MyGame.Sample;