Parse AsciiDoc files with any extension by default, following a Convention over Configuration approach

[manoelcampos]

If the adoc_file_ext param is not provided, the action should build all AsciiDoc files using find . -name "*.adoc" -or -name "*.ascii" -or -name "*.asciidoc".

If the param is given, build just files with provided extension. The param must not have a default value anymore.

[manoelcampos]

That will avoid questions such as #32.