Add "$JAVA_OPTS" to enable jvm options

[ackintosh]

This PR closes #9

[ackintosh]

I've checked that the option '-Dlog.level=warn' is respected. 😉

$ npm run build:all

> @openapitools/openapi-generator-cli@0.0.7 build:all /Users/akihito1/src/github.com/ackintosh/openapi-generator-cli
> npm run clean && node bin/build-all


> @openapitools/openapi-generator-cli@0.0.7 clean /Users/akihito1/src/github.com/ackintosh/openapi-generator-cli
> rimraf ./dist

Build package for openapi-generator-cli@4.0.0-beta2

...
...

$ env JAVA_OPTS='-Dlog.level=warn' ./dist/4.0.0-beta2/bin/openapi-generator \
          generate \
          -i https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml \
          -g php \
          -o /tmp/test


[main] ERROR o.o.codegen.DefaultGenerator - can't open 'php/.gitignore' for input, can not write '/tmp/test/.gitignore'
[main] ERROR o.o.codegen.DefaultGenerator - can't open 'php/.gitignore' for input, can not write '/tmp/test/.gitignore'

[kay-schecker]

thanks a lot, merged and released in v0.0.8-*