Adds make_warnings_subtle setting

Alternative fix #89

Setting `"make_warnings_subtle": true` in your
SublimeLinter.sublime-settings will now cause warnings to not
display when using the Show All Errors command.

This is the same command used when `"show_errors_on_save"` is set
to `true`, so enabling it will also cause warnings to not show
in the popup errors panel on save.
renamed again to `passive_warnings`
Slightly better labeling
reorder default settings and passive warnings false

By default, warning and errors should show up in popup
errors panel.

Closes #149
Closes #89