added option to select the journal in the zipfile to export

[sulrich]

• adds a command line option to specify the exported journal (in the zipfile) to convert to markdown.
• creates a journal specific export directory

[jbmorley]

@sulrich Thanks for the PRs! I'm a little busy with the holidays, but I'll take a look in the next few days.

In advance of that: it looks like you've used an automatic code-formatter and there's a fair few whitespace changes–I'd really appreciate it if you could keep the PRs down to just the functional changes. Thanks!