Add tab completion for CLI switches and task names

It would be nice to have tab completion for CLI switches of Cake .NET Tool and Cake Frosting.

Since `--target` is not a CLI switch for Cake .NET Tool implementing tab completion for task names might be challenging. For Cake Frosting `--target` is a CLI switch and it should be possible to implement tab completion also for task names. Performance will be an issue here, since we don't want to recompile on every key stroke and we should cache available task names somewhere (which would mean that task names are only available after first compilation).

Review the CLI handling for Cake .NET Tool and Cake Frosting, especially switch completion and task-name discovery around --target. Work out how available task names can be cached so completion does not recompile on every keystroke. Done means the requested completions work with the stated first-compilation limitation.

Written by the indexing model from the issue text.