Support markdown style link copying

[ifree]

This pr is for feature requests like #225. It serves my needs well, as it may be helpful to others hence I'm submitting it.

It's a quick fix, don't know If it meets the original design goal or needs more refactoring. Let me know if you have other concerns.

[czerwony-brudnopis]

This feature is precisely what I was looking for! Would you be so kind to tell me how I can pack your code into .xpi file?

[qqobb]

@ifree: Thanks for the PR! I haven't looked into it yet in detail, but I think you might want some customization option for the link text (#243). You'd also want to make the collection path in zotero://select links optional (#105). I'm therefore a bit skeptical about adding this function in its current form.

@ifree, @czerwony-brudnopis: Have you tried using an export translator? This is explained in #225 (comment). The advantage is that it's easy to adapt to a user's specific needs.