Add daily precipitation feature.

[asandhu3]

I think this adds a valuable piece of data to the forecast section. I've been using this for a while and thought I'd make some proper flags and logic for different units and throw it into a PR if you think it's useful to add. I attempted to add a 16x16 icon beside the number to indicate what it is but it looked a bit cluttered and the very small icons don't render too well IMO.

This is my first exposure to C++ so possibly some issues but feel free to review and offer suggestions!

[lmarzen]

Merged. @asandhu3 Thank you for your contribution!

I altered your PR to use the same PRECIP_UNITS as everywhere else instead of separate DAILY_PRECIP_UNITS. I also (unrelated to your changes) cleaned up the outlook graph while I was at it to only draw the precipitation y-axis labels when precipitation is forecasted since I got the idea from how you only display daily precip if it is actually forecasted.

You did a great job, made this feature ON by default.

[asandhu3]

Awesome, thanks!

[asandhu3]

I'm noticing one side affect of your changes that might impact the usefulness of this feature. I remember now why I had separated the units into their own config.

I used this feature to display POP on the graph, and mm on the weekly overview. That way I could at a glance see:

1. Which days of the week it's expected to rain (and how much), and
2. At what time of the current day it's expected to rain (on the graph)

By tying those units together it doesn't make that possible.

[lmarzen]

Whoops, should have discussed before changing it. I'll add that functionality back.

[lmarzen]

Reverted that change 3fc7cb1