Fixes #74

* The style functions are now more permissive about color names in the `color` argument. Color names should be valid CSS classes that can also be used as CSS variable names, but the `style_` function only stops with an error if the color name contains spaces. (@jdlom #69)

* Increment version number

* Better horizontal alignment of code blocks

* Use testthat 3e

* Use expect_known_output() because we need the css files

* Replace `with_mock()` with `mockery::stub()`

* Remove extraneous line

* Set ml and mr for all horiz alignment classes

* Add NEWS for #68

Fixes #53

Fixes #50

Fixes #41

Fixes #44

https://github.com/yihui/xaringan/pull/271

Avoids note for long argument length and allows for more consistent checking if supplied arguments are default values

E.g. primary_color and secondary_color in style_duo() become --primary and --secondary

style_* returns name of file invisibly

And add tests

Fixes #24