Improve compatibility of fonts between iOS and Android

Proposal

Drop support for setting fonts, instead support only the native system font (SanFrancisco & Roboto) but with size (existing), `font-weight` (new), `font-style` (new) options. Example:

```
"style": {
"font-weight":
"font-style":
}
```
where <style> would be one of `normal`, `italic`
where <weight> would be one of

```
UltraLight
Thin
Light
(default) Regular
Medium
Semibold
Bold
Heavy
Black
```
Then Jasonette would make a best-effort at mapping these onto system support font weights. Currently supported Android Roboto font weights:

```
Regular
Italic
Bold
Light
Thin
Condensed regular
Condensed bold
```

No files, tests, or entry points are named. Start by comparing the existing iOS and Android font-setting behavior, then define the native-font weight and style mapping and verify that the proposed options behave consistently on both platforms.

Written by the indexing model from the issue text.