Declare task dependencies in nimble file
It should be possible to have tasks depend on each other, so that invoking a nimble taks with dependencies would prompt nimble to first execute all dependent tasks first.
* if a dependant task fails, the entire invocation fails
* if multiple tasks in the execution graph depend on the same task, that task should only be executed once

To cause least conflict with exiting `.nimble`-syntax I propose, the following alternatives to declare dependencies:
``` nim
dependantTask build, makeBinDir
dependantTask test, [makeBinDir, build]
```
When dependencies are specified as an array or sequence, the order should be taken into consideration (however, task dependencies take precedence)

Alternative name for lazy typing could be: `depTask`. Alternate name for better explicitness could be: `dependencyFor` or `dependenciesFor`