provide context in the form of unchanged lines or AST nodes

**Is your feature request related to a problem? Please describe.**
When using diffsitter as tool for `git diff`, line numbers don't make it immediately obvious where the change occurred.

**Describe the solution you'd like**
It would be nice if some surrounding context (3 lines before, 3 lines after) could be given, similar to how regular `git diff` and `diff -C3` do it.

**Describe alternatives you've considered**
An alternative would be to allow machine-readable types of output (eg. `json`) for me or anyone else to write a new front-end around the logic of diffsitter.

**Additional context**
If it's not as obvious what counts as a line in AST-land, I would also be happy if I could get 10 AST nodes before and 10 AST nodes after the change (or some other configurable amount).