Setting/lint_mode to prevent linting on_load

[braver]

Originally posted in #1222 by @kaste:

I would like to see a change here for background mode as well and change the behavior of the BackendController.

We can have three modes (background|save|manual) plus a flag lint_on_load(true|false). Basically I could set background and lint_on_load to false for navigating through a code base, and linting would begin when I begin to write code.

EDIT: Removed outdated stuff.

[kaste]

This would help me a lot/sometimes. I don't know if we can do this as easy since we have the reason and lint_mode per project/linter thing.

The use case is simple:

• Just browsing reading a code base.
• E.g. Find/replace. If you do this on more files and forgot to disable SL ... you hate yourself