Fix copying of multiline snippets (#981)

For instance, the macOS build command

Author: hauntsaninja

conf.py

@@ -107,3 +107,5 @@
 # Strip the dollar prompt when copying code
 # https://sphinx-copybutton.readthedocs.io/en/latest/use.html#strip-and-configure-input-prompts-for-code-cells
 copybutton_prompt_text = "$"
+# https://sphinx-copybutton.readthedocs.io/en/latest/use.html#honor-line-continuation-characters-when-copying-multline-snippets
+copybutton_line_continuation_character = "\\"