Added support for .clang_complete files for consistency with vim and atom plugins

[haxiomic]

This PR adds support for .clang_complete files which are used in the atom and vim plugins

The advantage of supporting a .clang_complete file is that if the project is being used with multiple editors then only one set of clang complete arguments needs to be maintained

At the moment the arguments are added to the current options, rather than overriding, so if a .sublime-project is also used, then the result will be a combination of the options.

Do you think it would it be better to have the .sublime-project options override the .clang_complete file?

I'm new to python and sublime plugin development, so although it works, if it's going to merge it'd be good if it's looked over thoroughly to make sure it's the best approach

[trishume]

The code looks good to me and I would appreciate this feature :+1: cc @lvzixun