Allow to filter executed tasks

[danielmitterdorfer]

Every time we only want to run a subset of the tasks that are defined in a track we need to resort to copy & paste and create a new challenge. This leads to uncontrolled growth of challenges and we should reduce them again to the bare minimum and instead allow users to define a filter which tasks get executed.

Examples:

• Execute only the tasks with the name index and term: --include-tasks="index,term"
• Execute only tasks of type search: --include-tasks="type:search"
• You can also mix and match these: --include-tasks="index,type:search"