Add shortcuts to frameworks

[atusy]

For example, sakura() as mini_document(framework = "sakura") and similarly, water, spcss, and others.
It might be a good idea to include some features by default (e.g., add ToC by default), however, the reasonable values changes by users and use cases...

I would like to wait minidown matures, and then consider about which features to be turned on by default.

see also: #21 (comment)

[atusy]

Thanks @eddelbuettel for suggesting variety of candidate names: water, water_document, mini_document_water.
If we include some features by default, then the difference is not only framework or theme.
So, I want some name that also infers the document is enriched version of mini_document.

(BTW, I'm busy these days, so my response may take some time...)

[atusy]

@eddelbuettel
I would recommend you use _output.yml to share your favorite settings among the Rmd files.
As far as I tried, package vignettes can also benefit it by prvoding vignettes/_output.yml.