If racer fails to suggest, fall back to word based suggestions

[Lokathor]

Racer/RLS/whoever (honestly I don't even care who) is unable to provide suggestions for things most of the time. That's fine. Failing that I'd at least like VS Code to be able to suggest other words in the same document. This is normally a default setting, but if "rust.racer_completion" is true then it'll only show racer results, even when racer uselessly finds nothing.

VS Code: 1.29.1
Rust (rls) plugin: 0.5.0

[Xanewok]

We'd have to somehow conditionally enable word-based suggestions, cc microsoft/vscode#22680