Allow custom diff implementation

It would be great if you could wrap your diff output implementation in a method that could be overridden in a plugin. It could be as simple as a method that has two parameters to represent the before and after files and a string return value that represents the HTML to display.

In our case, we'd like to use the command line driven report output available in BeyondCompare. It provides tons of configuration options and our reviewers are very familiar with the appearance of the output.

We'd want this customization to apply to all diffs in the system--history comparisons, commit file list, pull request diff, etc.

I appreciate that a built-in side-by-side or configurable diff is a feature Atlassian isn't prioritizing at the moment. However, what I've requested above would allow clients that want an improved output to put forth some effort immediately to achieve what we want.