Add System Import/Export

[ssddanbrown]

[davidtorcivia]

Adding support for this feature. The export ability for single pages is great, but being able to export an entire book or chapter to PDF/HTML/TXT (or even EPUB directly) would be fantastic and could really expand the usecases of the software.

[patoroco]

Related with #177 for export whole book.

Anyway, I would like a feature to bulk-import pages from... (in our case) github wiki pages (a bunch of markdown files)

[viewty]

Markdown export would be great if it's possible for that to be incorporated at some point?

[terxw]

Currently I am using md text files and I am testing bookstack, it would be awsome to import all files from directory as pages.

[Leon99]

Any chances for this to get done? Migration scenarios are currently not possible, which means that BookStack missing out on all those businesses around with existing wikis.

[schuepp]

.md import would be great as a starting point for migration. PDF export of shelves would also be great.

[edenpc]

+1 for importing .md