[Feature Request] Import books: Allow mapping of custom metadata

[beedaddy]

What problem or limitation are you encountering?
In Calibre, I maintain metadata very carefully, for example the page cound. Unfortunately, BookLore cannot fetch this metadata field because it's a custom one.

What solution or improvement do you propose?
Let's the user define some of those custom fields and map them to well-known BookLore-fields. For example, #seiten could be mapped to the booklore-pages field.

Have you found any workarounds or alternatives?
Only workaround is to fill in those fields manually, which could be cumbersome for hundreds of books.