Smarter parsing of AsciiDoc attribute value passed from CLI
When an attribute value is passed from the CLI, it's parsed as a single YAML value. However, this behavior has unintended side effects. First, if the value is not valid YAML syntax, it will cause Antora to crash. If the value cannot be parsed, it should be treated as a string instead (taken as is). Additionally, if the value is surrounded in quotes, the value should be used as is. Otherwise, there is no way to pass in a value surrounded in quotes. The intention is only to be able to coerce certain data types to a concrete type, such as true and false or a number. However, we may even want to rethink that. Perhaps the value should just be interpreted as a string.
Parsing the value as YAML causes problems with the Maven plugin, which has to pass attributes to Antora through the CLI.