Add support for custom format

[louisld]

I tried to implement custom format for zotero links as described in issue #13.

Tell me if it suits your project of if you want to change some behaviour.

[jannessm]

Hey, sorry that it took so long for me to give feedback. I haven't tested it yet, but thank you very much for your effort!

Could you please change the following:

• Remove the Boolean flag and add a default value as the current template
• do not pass all the settings to zoteroItem. Just the ones it needs.
• I would suggest to only apply the template inside of [] not the whole replacement, but I am open for discussion.
• I would add a month option

Again: thank you!

[jannessm]

Another suggestion: please add the opportunity to format the date using normal date formatting syntax, e.g.: \<date:YYYY-MM-DDTHH:mm:ss>