Vim: drop 'foldlevelstart'

When I started using folds, I was worried that I'd forget to use them
and would fail to learn them unless I had a visual queue that they
existed.

So, I set some but not all folds to be visible whenever I entered a new
buffer by setting `'foldlevelstart'` in d211a0903989 (July 2010). I
think I've learned that Vim can do folding now.

Since then, I have pressed "zR" countless times (to reduce the foldlevel
to 0) on entering a new buffers. (Ironically, I should have been using
"zi" to toggle as it is easier to type and easier to say, which would
have come in handy when pair programming, when nobody else on my team
used folds.)

This is especially true when wanting to quickly look at all the files in
a directory in NERDTree using the "go" mapping (unlike "o", this opens
the selected file without shifting focus). So I would press "go" and if
the stuff I wanted to see was invisible due to folds I'd have to move to
the other buffer and hit "zR" (I mean "zi"), or give up on "go" and use
"o" followed by "zR"/"zi" and then switch back to NERDTree.

I think enough is enough.

I now have an easy way to toggle folding on anything I'm looking at (my
"<space>" binding, which I've got mapped to "za"). I have "zR", "zM",
"zr" and "zm" etched into my brain and muscle memory (and hopefully soon
"zi" too).

Time to enjoy the fruits of my lessons and delight in the efficiency of
a new work flow: most of the time, on landing in a file, I'll want to
get a sense of what's in it at a glance; if the file is too long or
dense to make sense of I can always hit "<space>" to get something out
the way, or "zm" (repeating as necessary) to zoom out and get a sense of
high level structure.

I probably should have done this sooner.