Add support for Asciidoctor skins

[ysb33r]

https://github.com/asciidoctor/asciidoctor-stylesheet-factory

As per @mojavelinux recommendation integrate with https://github.com/darshandsoni/asciidoctor-skins

[mojavelinux]

I really want to move people away from the stylesheet factory because it's not really maintained except for producing the default stylesheet. And that build is likely going to moved into core.

Better would be to integrate with the following themes that extend from the default stylesheet: https://github.com/darshandsoni/asciidoctor-skins

[ysb33r]

Conceptually a DSL like this might work:

plugins {  
  id 'org.asciidoctor.jvm.convert' version '1.2.3'
  id 'org.asciidoctor.html-skins' version '1.2.3'
}

asciidoctor {
   htmlskin {
      skin  'clear'
   }
}

(It should also be possible to use the org.asciidoctor.js.convert plugin instead of the org.asciidoctor.jvm.convert task)

A possible implementation:

• When plugin is applied the htmlskin extension is added to all org.asciidoctor.gradle.jvm.AsciidoctorTask and org.asciidoctor.gradle.js.nodejs.AsciidoctorTask instances.
• The addition of the extension will also add an attributeProvider to the asciidoctorj or asciidoctorjs task extension. The attributeProvider will add stylesheet attribute, but only if a value is actually set for htmlskins.skin.
• Since attribute providers do not affect upToDate status an task property will be required for when the skin name is changed.
• The resources will have an additional copySpec added, to copy the appropriate CSS files.
• Similar to pdfThemes a project extension will be added to take care of downloading the skins. In most cases this will not be need to be set unless the script author want to point to another repo or commit identifier

[jagedn]

It can be a great feature

My only concern is that it's seem different to pdfThemes. Maybe a similar syntax to pdf. Something as:

htmlThemes{
   local 'myname', {
       styleDir=file('....')
       styleName='bot-orange'
  }
}
asciidoctor{
   theme 'myname'
}

[mojavelinux]

FYI, as of Asciidoctor PDF beta.1, it's now pdf-theme and pdf-themesdir. (We're dropping the term style in this context).

[ysb33r]

This part is easy

htmlThemes{
   local 'myname', {
       styleDir=file('....')
       styleName='bot-orange'
  }
}

and it will be there, but this is not

asciidoctor {
   theme 'myname'
}

The reason is that at this point in time I would prefer to inject this as a task extension when the plugin is applied. If the above is to be supported I have to build support directly into the org.asciidoctor.gradle.jvm.AsciidoctorTask and org.asciidoctor.gradle.js.nodejs.AsciidoctorTask. It will also mean that instead of a separate plugin, htmlThemes will be available whenever org.asciidoctor.js.convert org.asciidoctor.jvm.convert` plugin is applied.

Maybe the latter alternative is feasible, I have just not been convinced whether this is better.

[ysb33r]

FYI, as of Asciidoctor PDF beta.1, it's now pdf-theme and pdf-themesdir. (We're dropping the term style in this context).

@mojavelinux See #438