Add support for link abbreviations (file only)

[mrvdb]

org-mode has a feature called link abbreviations:

http://orgmode.org/manual/Link-abbreviations.html#Link-abbreviations

The file-specific form of them looks like:

+LINK: example http://example.com/something

The defined link can be referenced in the text using square brackets:

[[example][Text that will get linked]]

I'm using org-ruby in combination with jekyll and for blog articles having a set of links organized together instead of scattered through the text would make the editting process quite a bit more enjoyable.

[wallyqs]

Hi, this was just fixed at #53
I will release a new version of the gem this week so that this can be used in Jekyll.

[mrvdb]

That was fast! Cool.