[lint addon] fix startLinting, onChange

[coderaiser]

When option lint set to jshint config and mode set to javascript, file.js is loaded - everything is OK.
But, if after that mode is changed to another one with no lint support (gfm for example) and set lint to false - after setting up value, errors are occur:

Uncaught TypeError: Cannot read property 'options' of undefined in startLinting
Uncaught TypeError: Cannot read property 'timeout' of undefined in onChange

[marijnh]

Good point. Attached patch solves the problem in a slightly different way (making sure the timeout is canceled when the option is reset). Could you verify that this works for you?

[coderaiser]

Everything works good, thank you. And it's definitely better to clearTimeout then just ignore it as I did.