Add fork option for mvn spring-boot:run

Update RunMojo to include a `fork` option and change the default
to only fork if agent or jvmArguments are specified.

Fixes gh-1412
Merge pull request #1483 from liujiong1982/spring-boot-1412

* spring-boot-1412:
  Add fork option for mvn spring-boot:run

Closes gh-1483