Added ability to specify paths for options files

I wanted to be able to use out of tree builds with cmake, and not have to worry about copying over the compile_commands.json file.  In this branch, you are now able to specify a filename to search for.  In my case, I can specify:

```
let g:clang_auto_user_options = 'compile_commands.json'
let g:clang_compilation_database_search_path = 'build/compile_commands.json'
```

which will properly find all the header files based on my cmake configurations. 

Now the the code isn't exactly perfect.  As you can see above, I kept how the clang_auto_user_options worked for backwards compatabilty and decided to add two new global variables for specifying the search path.  The other variable is for the .clang_complete search path (g:clang_complete_search_path).  Im not sure if changing the path for .clang_complete is as helpful but I figured we should keep the logic the same.