1.Rename completers

topydo.ui.prompt.TopydoCompleter is now topydo.ui.prompt.PromptCompleter
topydo.lib.Completer is now topydo.ui.CompleterBase

2. Reuse CompleterBase code in prompt completer.
3. Sort completion suggestions
4. Introduce completion of due: dates in column completer.
5. Store subcommands for completers in cache (via lru_cache).