Add single argument version of ContentNode#hasAttr

[mojavelinux]

Add a single argument version of the method ContentNode#isAttr. The purpose of this method is to determine whether an attribute is set, regardless of the value. For example:

document.isAttr("sectnums");

The method will also need to accept a boolean flag to control whether to consider the inherited attribute (which is true by default).

node.isAttr("target", false);

If this method conflicts with other overloaded methods, we could consider changing the prefix to has.

document.hasAttr("sectnums")

This may actually be a better approach. This is consistent with the DOM (which uses the method name hasAttribute).

[mojavelinux]

@robertpanzer if you like hasAttr instead of isAttr, feel free to update the title of the issue.

[mojavelinux]

In Asciidoctor.js, we decided to use more formal names, like getAttribute and setAttribute. I wonder if we should do the same in AsciidoctorJ 1.6.

[robertpanzer]

I would be in favor of doing so.
Method names isAttr and getAttr don't feel very Java-like.

[mojavelinux]

Exactly what we concluded for Asciidoctor.js. cc: @Mogztter