Add setting to force the use of multi-line cards

[gsingh93]

Multiline cards were added in https://github.com/eyeinsky/org-anki/issues/48. It seems like the code attempts to figure out itself whether the fields of a card should come from the headings. One problem I've come across is that this doesn't work well with org-anki-sync-all, which will sync all of the subheadings as new cards.

To fix this, there could be a setting to configure forcing multi-line cards. Or alternatively, maybe an org-anki-sync-all-multiline could be written to handle this case. I'm not sure what's a better solution.

[gsingh93]

I figured out I could achieve what I wanted by binding a function to org-anki-skip-function. I think this issue can be closed once this is documented.

[eyeinsky]

Multiline note front is supported when the Front or Name field is taken from a subheading, otherwise it is a single line (because org entries can't have multiline headings).

Generally (I guess this is the part that should be added to documentation), note fields are taken from subheadings in the case where the title of the sub-entries matches note fields. E.g something like this:

* Top level heading
** Front
multiline
front
here
*** Back
Back has always
been able to be
multiline

But the following wouldn't work:

* Top level heading
** Note front # this will become a note front with text "Note front"
multiline
front
here
*** Note back # this will become a new note with front as "Note back" !!!
Back has always
been able to be
multiline