feat: Add the DiffUnified() method for formatting a unified diff.

This adds a new `DiffUnified()` method which takes a `[]Diff` slice and formats
it in the "unified diff" format. The `[]Diff` slice does not necessarily need
to be a linewise diff.

The implementation deals gracefully with missing newlines at the end of either
side, as well as empty inputs on either side.

The method accepts two optional arguments: `UnifiedLabels()` for setting the
labels for the two inputs and `UnifiedContextLines()` for setting the number of
context lines printed.

Example usage:

```go
unified := dmp.DiffUnified(diffs,
	diffmatchpatch.UnifiedLabels("text1", "text2"),
	diffmatchpatch.UnifiedContextLines(3))
```

A convenience method, `Unified()`, takes two string inputs and returns the
unified diff.

Fixes: sergi#124