Add output filename and extension options

[honzahommer]

Desc in README

[benhughes]

Hey @honzahommer, thanks for the PR, why would you need to change the filename? the extension I can understand, but if you set the filename files will just be overwritten.

[honzahommer]

Hi, I am understanding you questing, but for better usage with e.g. GitLab pages you need to rename filename to index.html to use it as index file of directory.