Preemptively validate configuration

[untitaker]

While working on #30, I saw that todoman just passes the raw configparser through the CLI. I think it would be better if there was a dedicated Config type that has all valid params as properties (which raise if somebody tries to set an invalid value).

khal uses ConfigObj for this.

[WhyNotHugo]

We might want to close this if we merge #59, though that would have to be a mayor release, since it's backwards-incompatible.

[untitaker]

agreed