Add ability to create codemirror with merge plugin, fixes #91

[CarlAmbroselli]

This let's you use the merge plugin by adding merge-view to the directive.

Example:

<ui-codemirror
    merge-view
    ui-codemirror-opts="options"
    ng-model="editCopy">
</ui-codemirror>

$scope.options = {
        orig: 'StringToCompareWith',
        lineNumbers: true,
        highlightDifferences: true,
        connect: true,
        collapseIdentical: false
}

[LeSuisse]

I needed to use the merge plugin so I have done some functional testing of this PR. As far as I can tell this seems to work.
I think it would be a good idea if these changes could be considered for integration.

[yannis-rossetto]

Hello,

Any news about this contribution ?

[goldserg]

I don't understand how to use merge plugin.

Both values (old and new) I receive from ajax ($resource), and on initialisation moment I have no values...
What I should do?

html

js
$scope.editorOptions = {
mode: 'xml',
lineNumbers: true,
highlightDifferences: true,
connect: true,
collapseIdentical: false
};

...GET {
$scope.leftVar = val1;
$scope.rightVar = val2;
$scope.editorOptions.origLeft = $scope.rightVar;
}

[goldserg]

I used hack. Wrap in ng-if and directive start only after ajax complete.