Supercharged substitution with :Subvert

The [abolish plugin][abolish] provides a command called `:Subvert`, which is like a supercharged version of Vim's built-in `:substitute` command. The `:Subvert` command is especially useful for changing singular and plural variants of a word, and for refactoring names that appear in `snake_case` and `MixedCase`.

*This is part two of a three-part series on Tim Pope's [abolish][] plugin.*

[abolish]: https://github.com/tpope/vim-abolish