Vim plugin fix string normalization option (#1869)

This commit fixes parsing of the skip-string-normalization option in vim
plugin. Originally, the plugin read the string-normalization option,
which does not exist in help (--help) and it's not respected by black
on command line.

Commit history before merge:

* fix string normalization option in vim plugin
* fix string normalization option in vim plugin
* Finish and fix patch (thanks Matt Wozniski!)

FYI: this is totally the work and the comments below of Matt (AKA godlygeek)

This fixes two entirely different problems related to how pyproject.toml
files are handled by the vim plugin.

=== Problem #1 ===

The plugin fails to properly read boolean values from pyproject.toml.
For instance, if you create this pyproject.toml:

```
[tool.black]
quiet = true
```

the Black CLI is happy with it and runs without any messages, but the
:Black command provided by this plugin fails with:

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<string>", line 102, in Black
  File "<string>", line 150, in get_configs
  File "<string>", line 150, in <dictcomp>
  File "/usr/lib/python3.6/distutils/util.py", line 311, in strtobool
    val = val.lower()
AttributeError: 'bool' object has no attribute 'lower'
```

That's because the value returned by the toml.load() is already a
bool, but the vim plugin incorrectly tries to convert it from a str to a bool.

The value returned by toml_config.get() was always being passed to
flag.cast(), which is a function that either converts a string to an
int or a string to a bool, depending on the flag. vim.eval()
returns integers and strings all as str, which is why we need the cast,
but that's the wrong thing to do for values that came from toml.load().
We should be applying the cast only to the return from vim.eval()
(since we know it always gives us a string), rather than casting the
value that toml.load() found - which is already the right type.

=== Problem #2 ===

The vim plugin fails to take the value for skip_string_normalization
from pyproject.toml. That's because it looks for a string_normalization
key instead of a skip_string_normalization key, thanks to this line
saying the name of the flag is string_normalization:

black/autoload/black.vim (line 25 in 05b54b8)
```
 Flag(name="string_normalization", cast=strtobool),
```

and this dictcomp looking up each flag's name in the config dict:

black/autoload/black.vim (lines 148 to 151 in 05b54b8)
```
 return {
   flag.var_name: flag.cast(toml_config.get(flag.name, vim.eval(flag.vim_rc_name)))
   for flag in FLAGS
 }
```

For the second issue, I think I'd do a slightly different patch. I'd
keep the change to invert this flag's meaning and change its name that
this PR proposes, but I'd also change the handling of the
g:black_skip_string_normalization and g:black_string_normalization
variables to make it clear that g:black_skip_string_normalization is
the expected name, and g:black_string_normalization is only checked
when the expected name is unset, for backwards compatibility.

My proposed behavior is to check if g:black_skip_string_normalization
is defined and to define it if not, using the inverse of
g:black_string_normalization if that is set, and otherwise to the
default of 0. The Python code in autoload/black.vim runs later, and
will use the value of g:black_skip_string_normalization (and ignore
g:black_string_normalization; it will only be used to set
g:black_skip_string_normalization if it wasn't already set).

---

Co-authored-by: Matt Wozniski <mwozniski@bloomberg.net>
* Fix plugin/black.vim (need to up my vim game)

Co-authored-by: Matt Wozniski <godlygeek@gmail.com>
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
Co-authored-by: Matt Wozniski <mwozniski@bloomberg.net>
Co-authored-by: Matt Wozniski <godlygeek@gmail.com>