Allow system wide or user specific configurations

Add support for system and/or user specific configurations.  With this patch
applied it is possible to overwrite Buck's process JVM heap size:

  cat > $HOME/.buckrc <<EOF
  export BUCK_EXTRA_JAVA_ARGS="-XX:MaxPermSize=512m \
  -Xms8000m \
  -Xmx16000m"
  EOF