Separate out regex testing into it's own function to allow for easy testing of files

I'm not sure if this is the direction you wanted to go with this, but I liked your library and thought I would try this approach first. 

I added a new function to make it easy to test if a file has front-matter or not.
```
if (yamlfm.hasFront(content))
   // Hooray, do something
else
   // Move along
``

I didn't add a test because I made sure that the original process worked with the new function and tests pass. 

Please let me know if you would like anything to change. Thanks!