Implement auto fetch metadata (issue #9)

[dalanicolai]

Implement and document issue #9 auto fetch metadata from Google and Amazon.com (currently requires Ivy).

TODO (after discussion)

• [ ] Use different selection tool (instead of Ivy)
• [ ] Auto refresh calibredb-serach buffer (implemented as comment, but currently does not work due to Ivy action "preview metadata" switching to metadata-preview buffer
• [ ] rename metadata-preview buffer (currently has different name)
• [ ] Implement more sources options (preferably customizable as adding sources it slows down the process)
• [x] Copy covers to book directory

I have implemented the last TODO point already. There is no option to choose between different sources for the covers (like in Calibre gui) because the calibredb wtih -c flag always downloads from the same source, no matter which plugin has been chosen to request the metadata.

[dalanicolai]

The last commit here replaces PR #8, because it fixes the autoloading issues there correctly (autoloading is required for the Spacemacs layer. It seems this last commit is merged within my previous PR to add auto fetch metadata functionality (I do not see how to seperate these PR's)