Closes#1.
This change allows users to add a configuration section to their slides,
which allows them to specify a different theme if they would like to
change the styles of the default theme.
A user can add
```
---
theme: ascii
---
```
To use an ascii theme or use `theme: ./styles.json` to specify their own
custom JSON theme using glamour styles.
Co-authored-by: Britton Hayes <brittonhayes@users.noreply.github.com>
