Add support for external diff commands (e.g. difftastic)

docs/Custom_Pagers.md

@@ -62,3 +62,25 @@ git:
 ```
 
 If you set `useConfig: true`, lazygit will use whatever pager is specified in `$GIT_PAGER`, `$PAGER`, or your *git config*. If the pager ends with something like ` | less` we will strip that part out, because less doesn't play nice with our rendering approach. If the custom pager uses less under the hood, that will also break rendering (hence the `--paging=never` flag for the `delta` pager).
+
+## Using external diff commands
+
+Some diff tools can't work as a simple pager like the ones above do, because they need access to the entire diff, so just post-processing git's diff is not enough for them. The most notable example is probably [difftastic](https://difftastic.wilfred.me.uk).
+
+These can be used in lazygit by using the `externalDiffCommand` config; in the case of difftastic, that could be
+
+```yaml
+git:
+  paging:
+    externalDiffCommand: difft --color=always
+```
+
+The `colorArg`, `pager`, and `useConfig` options are not used in this case.
+
+You can add whatever extra arguments you prefer for your difftool; for instance
+
+```yaml
+git:
+  paging:
+    externalDiffCommand: difft --color=always --display=inline --syntax-highlight=off
+```