Add configurable notebook name pattern

[JeppeKlitgaard]

I have a use-case where I need to distinguish between manually written markdown files and those generated by nbconvert, which I would like to do by adding a 'pre-extensionsuch thatnotebook.ipynbwould be written tonotebook.generated.md`.

I suggest changing the output_base configuration option of NbConvertApp such that this may be used with multiple inputs and can contain pattern information using a handlebars format similar to NbConvertApp.output_files_dir (Default: {notebook_name}_files) as detailed in docs

The following two handle variables should be defined:

• {notebook_name} which is the base name of the notebook file
• {index} which is the one-indexed position of the notebook in the self.notebooks attribute of the NbConvertApp instance

I believe this change could be made backwards-compatible for all configurations except those containing either of the two handlebar expressions in the output_base option, which I presume no real-world configurations have.

I would be willing to provide a PR adding this functionality