Allow custom extensions when editing (for easier syntax highlighting)

[KarimPwnz]

With this PR, the temporary filename's suffix will either be its template filename or ".jrnl". This allows custom extensions and formalizes the ".jrnl" extension for "better text editor integration." I have also created the necessary tests.

This closes #1059 and closes #1080.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have tested this code locally.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.
• All tests pass.

[wren]

This looks fantastic. Thank you!

[alichtman]

Has anyone written a custom syntax highlighting package for vim / neovim for .jrnl files? Is this in the works?

[wren]

@alichtman I have a modified markdown sytnax that I use in my personal dotfiles for jrnl (see here). But there's not a proper package that I know of. We're not currently planning one, but would be happy to accept PRs and/or link to a package in the docs.

[alichtman]

Ever considered making your syntax file a vim plugin? Basically just drop it in a separate repo, and make it importable through plug-in managers. I'd use it.

[wren]

The reason I've put off making it a plugin is because the implementation is brittle, and tailored to my personal preferences. I keep telling myself that I'll clean it up and making a plugin, but haven't made it a priority.

But I think this might be an instance of the perfect being an enemy of the good(ish) enough. If I make it a plugin now, then users would likely file issues and PRs that would help improve it for everyone.

So, why not, I guess. I haven't made a vim plugin before, but I'll give it a shot with this. Thanks for prompting me about it!