Per-project executable setting

[ajmichels]

According to @aparajita in SublimeLinter/SublimeLinter#32 (comment) each linter package needs "to add an “executable” setting... that would be honored per-project".

It would be great if we could get that support in this package.

Being able to specify something like this in project settings would be great:

{
    "SublimeLinter": {
        "linters": {
            "phpcs": {
                "bin": "${project}/vendor/bin/phpcs"
            }
        }
    }
}

I am working on several projects which use different versions of linters and having this support is critical as updating the global settings or my environment PATH every time I switch between projects is not realistic.