Adding mass addition of bib information feature

[u7485624]

Begun working on Issue #372 - #372

Just added some basic functions to a class. Need some help figuring out the exact guidelines/functionality that the feature requires. Do we assume that the ID (DOI/arxiv) is given or do we extract it from a given URL etc?
If so, can we just use pattern matching from the URL to determine the IDs?
In terms of the better implementation, do we compare extracted entries from DOI/arxiv and then take the longer string?

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

No need to re-implement fetching from arxiv. JabRef already has arxiv support. You "just" need to wire the existing fetchers together in a smart way.