Automatically set stylesdir if style ends with .yml

[mojavelinux]

If the style is a path (indicated by the fact that it ends with .yml), automatically set the stylesdir to the directory of that path if not explicitly set.

For example, the following command:

asciidoctor-pdf -a pdf-style=/path/to/my-theme.yml source.adoc

would set the stylesdir to /path/to.

By doing so, resources in the theme file, such as images, will automatically get resolved from this directory.

[mojavelinux]

Here's the logic to use:

• if the style is not set, use the built-in default theme and set the stylesdir to the built-in stylesdir
• if the style ends with .yml and stylesdir is set, resolve the theme relative to the stylesdir and keep the stylesdir as is
• if the style ends with .yml and stylesdir is not set, use the theme at the path specified and set the stylesdir to the directory of that path
• if the style does not end with .yml and stylesdir is set, append -theme.yml to the style value, resolve the theme relative to the stylesdir, and leave the stylesdir as is
• if the style does not end with .yml and stylesdir is not set, append -theme.yml to the style value, resolve the theme relative to the built-in stylesdir, and set the stylesdir to the built-in stylesdir

[mojavelinux]

Btw, we're also proposing to rename pdf-style / pdf-stylesdir to pdf-theme / pdf-themesdir. See #1127.