4 choices; YONV variant

I've just started using local_vimrc.  It rocks!

Here's an enhancement.  The answers it accepts to its prompt are asymmetrical.  Only *yes* is "sticky"; if you say *no*, it asks you again the next time, and the next, ....

Basically, there are two independent questions encompassed by that prompt:
  - Read the file this time?
  - Remember the answer for next time?

yielding, of course, four possible combinations, of which only two are implemented.

This branch implements the other two.

I actually came up with two variants.  Implementations are identical; it's just the names for the choices offered to the user that are different.

One variant, which I'll refer to as **YONV** for obvious reasons, gives the user these options:
  - *Yes*: read the file; sticky
  - *Once*: read the file; not sticky
  - *No*: don't read the file; not sticky
  - *neVer*: don't read the file; sticky

That's what's in this PR.

The other variant, **YNAV**, is:
  - *Yes*: read the file; not sticky
  - *No*: don't read the file; not sticky
  - *Always*: read the file; sticky
  - *neVer*: don't read the file; sticky

The advantages of YNAV are that *yes* and *no* "match", in that neither one is sticky; and the words *always* and *never* have nicely matching sticky meanings to go with the sticky choices they represent.  The down side is that *yes* becomes non-sticky, which isn't backward compatible.

YONV *is* backward compatible; it preserves the current meanings of both *yes* and *no*.  Plus, I kind of like *once* as the name for its option.  But *yes* and *no* remain asymmetrical -- one sticky, one not -- which goes against the grain for me.

In the end, I can't decide which I prefer.  So I'm offering you both choices ... um, meta-choices I guess :-)  Assuming you like this idea at all, please pick one of the PRs to review, and just close the other one.  And speaking of review, this is my first attempt at nontrivial VimL, so please review it especially carefully.

Thanks.
  - Eric