Set outfilesuffix attribute to extension of source file

Resolves #46 and improves #45 
Unfortunately, the mutable part doesn't work. Internally, Asciidoctor overwrites the value of this attribute if it is not "locked". An attribute is locked when it is set from the API without the trailing "@".

Here's an example you can use to test the feature. The following inter-document cross reference should resolve to "other-file-name.adoc#other-section-name".

```
<<other-file-name#other-section-name>>
```

Ultimately, I think Asciidoctor core should honor the mutable outfilesuffix value, but that depends on https://github.com/asciidoctor/asciidoctor/issues/692 in order to distinguish where attributes are set at a more fine-grained level.

In reality, I can't imagine anyone needing to set outfilesuffix in the Chrome extension, and using the file extension of the current file is the most sane default.