Add support for configuration and removed executable check

The primary purpose of this request is to add support for configurations to allow you to set a pylint executable and pylintrc. This would solve issues #16, #14 and supersede the pull request in #21. It is important to be able to switch pylint executables because many times Linux distros include both python 2 and 3 now.

I removed the executable check for three reasons.

1. It decreases readability and adds unnecessary complexity to debug in the event that linter changes
2. As far as I can tell no other linters do this
3. Most importantly it causes the linter to silently fail from the perspective of the user. Every other linter will generate an error in the interface if the executable doesn't exist, while the error generally isn't very descriptive it at least lets the user know the linter failed. Since it is possible to change the pylint executable now if they enter a wrong executable they should be alerted.