Add function getLineDiffDetails

Function for retrieving the details of the diffs in a file. It works the same as `gitLineDiffs`, but uses the `git_diff_line` callback. I extracted the blob retrieval part into its own reusable function.

A package for viewing the diffs inline is already in the works [here](https://github.com/samu/git-diff-details). Needs the changes from [this](https://github.com/samu/atom/compare/update-repo-api) branch.

Yes, it could be an option to `getLineDiffs`. Here are the reasons why i implemented it as it's own function:

* `getLineDiffs` essentially returns information on all the _hunks_ that have been modified. Say we have a file with 3 hunks with arbitrary counts of lines, we get a list with 3 objects. The `getLineDiffDetails` version though will return a list of much more objects, one entry per line. I think the API of these two cases differ too much in order to come from the same function.
* In the case of the `getLineDiffDetails` function it might also be interesting to allow setting the `context_lines` option. This is not true for `getLineDiffs`.