Improve Diff Handling

I aim to improve the existing diff handling capability of coala so that it is possible for bears to offer multiple fixes for an issue. I also plan to implement interactive diff behavior where the user input is used to generate interactive diffs using appropriate placeholders in the code. For achieving this I plan to have a generic approach for bears to pass their own applicable actions which can then be used to write a generic bear which can offer multiple patches. This can be followed by Implementation for templated patches and interactive diff behavior along with writing tests supporting these new functionalities. Finally thorough testing and exception handling should be done to fix bugs and designing alternate approaches if necessary(e.g. resolve conflicts of multiple bears on a single issue by git like conflict markers through IDE). I would be updating the documentation throughout the project. An optional task would be implementing output formats for the enhanced features and their processing functions being implemented in the Linter class if time permits.