Periodic linting of the entire file

[fwcd]

Lint the entire file either once the user stops typing or periodically in the background to prevent inconsistent diagnostics (see #47 and #38).

[casret]

Yeah, great idea. I have a lot of problems with old warnings not going away.

[fwcd]

The linter is now triggered every time the document is saved. So if there are any inconsistent diagnostics, just re-save.