<specFlow>
<unitTestProvider name="..." /> <!-- this you have already -->
<generator allowDebugGeneratedFiles="true" /> <!-- this is the important setting -->
</specFlow>
This is the configuration Gaspar Nagy has recommended in the App.config file. The "allowDebugGeneratedFiles" seems to be the one ticket. More info on the tag and it's sub-tags can be found on the official SpecFlow configurations page. Setting it to TRUE is only the first step though.
As Gaspar suggests, the next step is to right-click on the project file in the Solution Explorer and to select the Regenerate Feature Files option.
NB: What actually happens when you set the allowDebugGeneratedFiles flag and then regenerate the .feature files, SpecFlow simply removes the #line hidden and #line number from the .feature.cs file.
Finally, I'd personally suggest to restart Visual Studio 2012 before or after rebuilding. When I tried the above steps, the Test Explorer showed the tests correctly only after restarting the Visual Studio.
PS: Gaspar noted that if the allowDebugGeneratedFiles flag is raised (=true), then using the "Run SpecFlow Scenarios" option won't work anymore. However, I was able to use that option by right-clicking on the project file. Using that option on a .feature wasn't working for me even before I tried Gaspar's suggestion so that must be the drawback that he was referring to.
PS #2: Gaspar also mentions that there won't be such a drawback will be removed in SpecFlow 1.9.2.