Adding styler support

[lorenzwalthert]

Finally, I got time to try to add styler support as suggested in #1516. I have the following questions:

• As of now, there are two options tidy (defaults to FALSE) and tidy.method (defaults to "formatR"). We could remove the tidy option. This would result in a backward-incompatible API change, which is probably not a good idea. I think it would make sense to simply issue a warning when someone sets tidy.method = "styler" and (either because of explicit specification or because of the default) tidy = FALSE, because he or she probably just wants tidy = TRUE and tidy.method = styler. I have not yet implemented that, so let me know if I should go ahead with that.
• I don't know what resources (e.g. documentation, cheat sheets etc.) need to be updated to reflect the API change, at least I could not find any help file where the tidy option was documented.
• Also, I wasn't sure about if and how I should add unit tests for this.

Could you please advise on these points? Thanks.

cc: @krlmlr

[krlmlr]

How about supporting tidy = "styler" and tidy = "formatR", in addition to TRUE and FALSE ?

[yihui]

@krlmlr Yes, I like the idea of using one option tidy = 'formatR' or 'styler'. To make it backward-compatible, tidy = TRUE means tidy = 'formatR'. We could even make this option accept a function, so that it is completely extensible.

@lorenzwalthert I'll just tweak the PR by myself and merge it. Thanks!

[lorenzwalthert]

Cool, thanks @yihui.