Add option to create Spring Boot CLI apps based on checkboxes

Currently, user can create maven/gradle projects/build files. Another useful option would be to create a Spring Boot CLI app.

Some things, like Web, might entail:
```java
@Controller // or use @RestController
class MyController {
}
```
But if the user picks Thymeleaf, we need this:
```java
@Grab("thymeleaf-spring4")
@Controller
class MyController {
}
```
And so on. Some things might manifest in other ways. And for checkboxes we don't yet have a decision, perhaps a comment like this:
```java
// You picked <foo>, but we don't yet support that here
...
```