Fixes, and a restructuring

Here are a few small fixes, plus a bigger final restructuring.

In the end result, here's how it works:

* The user never enables the minor mode directly
* The user calls `om-insert-template`, which is the (renamed) skeleton function
* After insertion, "om-insert-mode" is enabled, which allows the use of `C-n` and `C-p`, with `C-c C-c` to exit

I tried it out and it seems to work okay -- perhaps take a look and see what you think.