Add optional document link prefix

[raphaelmenges]

Hello @rjoberon,

This pull request regards the proposed changes in #4 and my pull request in the bibsonomy-ruby plugin.

Cheers, @raphaelmenges

[coveralls]

Pull Request Test Coverage Report for Build 33

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at ?%

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Totals
Change from base Build 32:	0.0%
Covered Lines:
Relevant Lines:	0

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💛 - Coveralls