Differentiate clear weather at night vs day

[galvanist]

I've had a go at fixing #68 (night should be moon instead of a sun).

This includes an ascii moon/stars for the ascii-art-table frontend and an appropriate character in place for the emoji frontend.

Only fixed for the forecast.io as the placeholders were already there, and I don't have access to any API keys for wwo.

Here's how it looks for a suitably sunny/clear location:

[schachmat]

Heyho @galvanist.

A few points of feedback:

• The differentiation between day icon and night icon should not be done by the backend, but just from the Time value inside the frontends. It's okay if you only implement it for the ascii-art-table frontend, as it is the default one.
• The iface condition codes should not be extended, but rather changed from e.g. Sunny to Clear, so they apply to both night and day conditions.
• There should be alternatives for all other icons in the ascii-art-table frontend which contain at least part of a sun.

If you want to pick up some of the points, you are most welcome to do so. Otherwise I will probably do it when I have some spare time. The PR will not be merged until the first two points mentioned above are done, the third one adding the missing icons can be done later.

Thanks for drawing that nice icon!

[schachmat]

Work on this feature continues in #115.