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>
Fixes#15
This change allows users to pipe in slides from `stdin` which allows for
`curl`ing remote slides and presenting them as well as piping multiple files
into one (with `cat`) and much more.
```
curl http://example.com/slides.md | slides
```
This change also maintains backwards compatibility with the current API which
allows for users to simply specify a file name. In the future, it would be nice
to remove that in favour of simply reading the file from stdin.
Like so:
```
slides < examples/slides.md
```
