Add justl-per-recipe-buffer custom variable

[TristanCacqueray]

Fixes #44

[TristanCacqueray]

Running the tests with and without the custom seem to work, though I am not sure how to test the setting actually work, e.g. can we set the custom for the duration of a given test?

Also the :sentinel arg does not seem to be used and since this change the api I think we should remove it.

[psibi]

@TristanCacqueray LGTM overall.

can we set the custom for the duration of a given test?

Not sure. But can you write a test such that you do customize-set-variable and then run a recipe and check that the new buffer is actually created ? And then at the end reset using customize-set-variable to the original value so that it doesn't affect other test.

Also, if you manually test to check that TRAMP integration is not broken using the following steps: https://github.com/psibi/justl.el/tree/master/test ? Alternatively, I can do that at the end before merging this.

One more thing: If you can update the changelog and README with an explanation of this defcustom, that will be helpful also.

Also the :sentinel arg does not seem to be used and since this change the api I think we should remove it.

As long as the test passes and the TRAMP integration doesn't break, go for it. :-)

[TristanCacqueray]

@psibi Thanks for the prompt review and the suggestions. I have validated that this change works over TRAMP, but using ssh instead of docker (because my emacs is already running in a container :).

[psibi]

Thank you!