Limit TODO.md search path

[anilanar]

I'm using this plugin full-time now, except that I had to fork it to remove a feature: crazy-deep TODO search path. At each directory level, it looks deeply for TODO.md files in every subdirectory and goes up to parent directory until it finds a TODO.md. That's very slow if the TODO.md file is a few levels up and if there are subdirectories such as npm's node_modules.

See the commit I made for myself here: https://github.com/anilanar/vim-getting-things-down/commit/05afff725ac399d28b64172c9c5b1c4759df6efe

[rlue]

This is one of them why-didn't-I-think-of-that situations!

Feel free to submit a PR.