Support warn for <attributeMissing>

[wilkinsona]

The Asciidoctor user manual recommends that warn is used for attribute-missing:

leaves the reference intact, but also prints a warning about the missing attribute (recommended)

It doesn't appear to be possible to use it via the Maven plugin with skip, drop, or drop-line being the only supported values:

warn is not valid. Must be one of 'skip', 'drop' or 'drop-line'

asciidoctor-maven-plugin/src/main/java/org/asciidoctor/maven/AsciidoctorMojo.java

Lines 558 to 562 in a4d86a9

	if ("skip".equals(attributeMissing) || "drop".equals(attributeMissing) || "drop-line".equals(attributeMissing)) {
	attributesBuilder.attributeMissing(attributeMissing);
	} else {
	throw new MojoExecutionException(attributeMissing + " is not valid. Must be one of 'skip', 'drop' or 'drop-line'");
	}

Could support for warn please be added?

[wilkinsona]

As a workaround, it can be set via <attributes> instead:

<attributes>
	<attribute-missing>warn</attribute-missing>
</attributes>

[abelsromero]

I did not see that when reviewing changes for 2.0.0, and I am more in favor of using the attributes. How would you feel if we removed attributeMissing and attributeUndefined from ?

[wilkinsona]

That sounds good to me. I wasn't sure if <attributeMissing> would override whatever I'd configured in <attributes> so I was pleasantly surprised that the workaround was possible. Having only one way to configure it sounds good to me.

[abelsromero]

This configurations are legacies from early stages of the plugin in which we offered many attributes as configurations before fully supporting all <attributes>.

I wasn't sure if <attributeMissing> would override whatever I'd configured in <attributes>

Attributes are applied after configurations, so they override any configuration.