Add tab-completion for Flags and Switches #15

Tab-Completion for Flags and Switches are a useful feature for advanced commands, but are still left out quite often because of implementation difficulties.

Suggestions should switch to flags/switches as soon as the user types the '-' symbol. All the available flag/switch names should be listed then. When a flag is chosen, the completion should switch be handled by the Part again. If it's a switch, it should just switch back to normal completion.