Support omitting checksum when generated-file-name is provided

[vy]

generated-file-name parameter is fixed with #48. Currently, [ditaa,foo] generates foo-cf699e6a13a1976b55764d80b6f9d508.png. It is pretty common that search engines and users link to these image files. Whenever I make a change in the diagram, the checksum will be updated, and the old filename will start giving 404s. Is it possible to exclude the -<checksum> part completely to avoid this? Or is there any other way to approach to this problem?

[ggrossetie]

I think we need configuration (AsciiDoc attributes) to control how the extension caches/writes data on the disk. I will probably need to revisit my initial design since there are quite a few issues related to the "cache mechanism".