Add text flow when generating diagram

If you not using default fontName (activity and label) and generate diagram manual outer engine (Spring MVC or Struts2 eg.),

```java
InputStream imageStream = ProcessDiagramGenerator.generateDiagram(bpmnModel, "png", activeActivityIds);
```
please use codes bellow ( use spring engine ):

```java
@Autowired
ProcessEngineFactoryBean processEngine;

Context.setProcessEngineConfiguration(processEngine.getProcessEngineConfiguration());
```
or standalone:

```java
ProcessEngineImpl defaultProcessEngine = (ProcessEngineImpl) ProcessEngines.getDefaultProcessEngine();
Context.setProcessEngineConfiguration(defaultProcessEngine.getProcessEngineConfiguration());
```