Allow custom file extension for jrnl --edit command

[rizaon]

Feature Request

Currently, jrnl --edit comand will open temporary .txt file using user preferred editor.
https://github.com/jrnl-org/jrnl/blob/4ee4f38/jrnl/editor.py#L15
It will be great if there is a config to specify the file extension to use other than .txt.

Use Case/Motivation

Some editor has support for text highlighting depending on what is the file extension, such as markdown-mode (.md) or org-mode (.org) in emacs.
Allowing user to specify the preferred file extension to use in this edit command will in turn let the editor to display the right text highlighting.

[wren]

This is a good idea. Should we add a config value. What would we name it?

Maybe we can just use the template field we already have? We could just copy the extension that the template has (and allow a blank template, too, in case people just want to give an extension without a full template).

[micahellison]

I also like using the template field for to resolve this.

We were just talking about this today and toying with the idea of using the whole template file name as a suffix to the temporary file name when editing. So if, for instance, a template is named mytemplate.jrnl the temporary file could look like: jrnlmf1p_rgj-mytemplate.jrnl

This would solve this issue, while also making temporary jrnl entries a little bit more identifiable in editors for people such as myself that have waaaay too many tabs open all the time.