Allow to include modifiers in the syntax to be used for multiple subtasks

[Stefan Glase]

Idea is to avoid multiple lines for every subtasks that need to be set repeatedly.

# component: iOS
- task with component iOS
- another task with component iOS

# component: Android
- task with component Android
# component: Backend
- task for component Android and Backend

Further considerations

• empty line is used to clear modifiers
• what do we do with "overrides" inside a block? Do they always override or do they work additionally on fields that allow multiple values (like tags)
• also accept multiple modifiers for blocks