Many of the plugins in vim-plugin-names are out of date and redirect to
their new github repos. This commit adds a flag to automatically update
these and defines a process for committing these updates into nixpkgs.

In response to @timokau's review here are a couple changes:

- Decrease the fragility of the replacement code by normalizing
whitespace on each line.
- Throw an error when plugins are renamed rather than silently aliasing
to the new name.

Per review, the input file will now be rewritten automatically if
redirects are found.

Python's `sorted` method works a little differently than `sort` in the
handling of dashes.

When an updated redirect results in a package name change, not just an
owner change, throw an error to warn users the package names has
changed.

VimPlugins: update redirected repos