Run RuboCop on unsaved (virtual) files

[mrcljx]

RuboCop runs on the files that is saved to disk - if you have unsaved changes, the RuboCop annotations might be confusing.

In an early prototype of the plugin I didn't use the files on the file-system but wrote the content of the current Document (saved or unsaved) to /tmp first and then executed RuboCop on that file. The issue here is, that the exclude setting in the RuboCop-config would be "ignored" or it would cause other unwanted behaviour.