project-wide import statement cleanup

[brandon1024]

IntelliJ has a feature to optimize imports for an entire project. We should implement similar functionality, recursing from the cwd.

[brandon1024]

If we take this on, it'l be important to ensure that we don't load all of these files into memory. That would be pretty nasty. I don't think this will be possible in the current version of the plugin without some changes, since this functionality currently relies on buffer.vim.

Additionally, it should be possible to provide a base path as an optional argument to limit the cleanup to a specific root directory.