Add command to run tasks only

[tmaczukin]

There should be tasks command similar to currently existing build or update.

Used without arguments should execute all tasks defined in local manifest (or also in all manifests of dependent components if --allow-nested-subtasks option was used). If arguments are present they should be treated as list of names of tasks to execute.

In the second situation - in my opinion - it should not be possible to use --allow-nested-subtasks option (or there should be some kind of namespaceing convention for tasks names).

Usage:

$ gitcompound tasks [--allow-nested-subtasks] [task_name_1 [task_name_2 ...]]

Examples:

1. Execute all tasks of local manifest

   $ gitcompound tasks
   Running tasks ...
   (...)
   

2. Execute all tasks of local manifests and all dependent manifests

   $ gitcompound tasks --allow-nested-subtasks
   Running tasks ...
   (...)
   

3. Run tasks prepare_cache_dir, symlink_assests from local manifest

   $ gitcompound tasks prepare_cache_dir symlink_assets
   Running tasks ...
   (...)
   

4. Run tasks prepare_cache_dir, symlink_assests from local manifest and show warning

   $ gitcompound tasks --allow-nested-subtasks prepare_cache_dir symlink_assets
   Warning: --allow-nested-subtasks option is disabled when tasks names are used.
            Executing only local manifest tasks.
   
   Running tasks ...
   (...)