Add feature to make deploying preview docs for a PR easier

[oxinabox]

@fredrikekre has code that works
fredrikekre/Literate.jl#75

But it is pretty evil,
It would be nicer if we could just say call deploydocs
with the right arguments.

[oxinabox]

Other issues:

• Old packages link to latest in readme, but this has been fixed in new version of PkgTemplates Use "dev" instead of "latest" to link GitHub pages JuliaCI/PkgTemplates.jl#52
• PR doesn't show up in version selector from stable/dev versions because the current hack doesn't update the versions.jl, though I might argue that is actually a feature.

JuliaDiff/ChainRules.jl#105 (comment)

[oxinabox]

It would be also good if we could have a way to have a comment automatically go back to the PR saying "Your docs have built, new URL is at"

[fredrikekre]

PR doesn't show up in version selector from stable/dev versions because the current hack doesn't update the versions.jl, though I might argue that is actually a feature.

Yea, hence the delete_method for that, didn't want "real" docs to be affected in any way.

[oxinabox]

seems legit, if we has it as a built in option, we might want to make it a kwarg:
"Include preview in main listing" or something?

[fredrikekre]

push_preview to deploydocs now implemented. Pushes to .../previews/PR$(NUM).