Specifify implementation for org-babel tangle

[Simen Endsjø]

When entering a begin_src scheme block, Geiser will pop up asking what
implementation it should use. By adding
;; -- geiser-default-implementation: guile --
to the code block body, it no longer asks what implementation to use.

When running org-babel-tangle, on the other hand, it asks the question for each
block, which is quite annoying, but also difficult if I’m using different
schemes in the file.

Any idea how to fix this issue?