Change profile section names in aws config file

Profile names are now specified as:

```
        [profile "myname"]
        aws_... = foo
```

instead of:

```
    [myname]
    aws_... foo
```

The motivation for this change is to "make room" for other
configuration options in the config file.  Currently, any section
name in the config file is considered a potential profile name.

Another alternative is to just whitelist a set of configuration
options that aren't profiles, but then that restricts what names
you can use for profiles.

As for the specific syntax, I don't feel that strongly about it.
We could also use `[profile.myprofile]`.  I picked this format
mostly because it's the same as git's config, and figured most people
are likely to have seen this format before than a `[profile.name]`
format.  But I'm not opposed to changing it.

I also kept the `[default]` section the same, though if we wanted to
be perfectly consistent, it should be `[profile "default"]`.  I figure
since most people would have a `[default]` section in their config files
that we shouldn't change it, but again not opposed to changing this
to `[profile "default"]`.

cc @garnaat @toastdriven