Default content to back field

[leoc]

This change lets the building of fields default to the back field for content that does not contain another child heading.

Regarding #61 and #59

Now we can write:

* My question for the front?
** Back
My answer at the back of the card...

as

* My question for the front?
My answer at the back of the card...

No need to specify a "Back" heading.

Maybe we can use "per-card-type" field names...

Cheers!

[gsingh93]

@louietan any chance you could merge this?

[searene]

Can this be merged into master? It would be really helpful.

[orgtre]

This commit in orgtre/anki-editor provides the functionality of this pull request. It also allows a more general logic:

• If exactly one field is omitted (based on the ANKI_NOTE_TYPE), then the content between the note heading and the first subheading (or the next same-level heading) is mapped to the missing field.
• Moreover, if exactly two fields are omitted, the note heading is mapped to the first missing field, and the second missing field is mapped as described in the previous sentence.