Let Abricotine open local links

[seanlucrussell]

Proposal to implement feature requested in #303. shell.openItem (shell.openPath in newer versions of electron) generalizes shell.openExternal and lets the OS open local links using the appropriate application. URLs will still open as they do now, but this will also allow links to pictures, pdfs, or indeed other markdown files to open in the same way.

Tested on Ubuntu 20.04 against these test cases:

1. [absolute path](/home/<user>/abricotine/README.md) - OPENS CORRECTLY
2. [relative path](abricotine/README.md) - OPENS CORRECTLY
3. [empty path]() - NOTHING OPENS AS EXPECTED
4. [website](https://github.com/brrd/Abricotine) - OPENS CORRECTLY
5. [website no proto](github.com/brrd/Abricotine) - OPENS CORRECTLY
6. [bad path](nonexistent-nonsense) - NOTHING OPENS AS EXPECTED

[seanlucrussell]

Throwing this proposal out there for three reasons:

1. In case anyone else wants to know how to implement this feature (for personal use, for future forks, et cetera).
2. In case the project does end up getting continued
3. Because I didn't read the pinned issue Contributing to Abricotine #310 (whoops!) until after I'd already made the change locally and I felt like it would be too much of a shame to leave it on my personal device. I've really been enjoying Abricotine for note taking, and linking between different notes is a really handy feature.

But I also understand the reluctance to continue the project (I hate getting trapped by my old code!), so feel free to close this pr at your pleasure.

[brrd]

Thank you, I'm merging this PR but I don't know when I will be able to publish a new release including this feature.