Use user configured textwidth if it is set #286

rust.vim version: rewritten-indent branch (unrelated to the issue)

Steps to reproduce:

1. Create a .vimrc which sets textwidth=80
2. Open a Rust file
3. Run :set textwidth -> it's at 99

Expected vs. actual behavior:

I expected the textwidth not to be changed by rust.vim.

It seems to be set here:

I think it makes more sense to only set it, if it is not already set.