add a default flag for custom build systems in sublime-project.json

I would like to be able to set a property such as `"default": true` when defining build systems within project files. Once this is set, then pressing ctrl-b within a project would automatically execute the default build process (unless you explicitly select something else).

Perhaps instead of a boolean, it could be a list of extensions or regexes: `"default": ["*.py"]`, so that the specified build system becomes the default for, in this case, only Python files. And you could specify something else for e.g. CoffeeScript files.

My particular use case is this: I have a project template with which I distribute a sublime-project.json file. Projects using this template are always going to use a virtualenv'd python at env/bin/python. So I specify this python instance in the project file's build systems. If there were a default flag, then I wouldn't have to remember to select this build system in the drop-down menu for each new project; it would just work.

This is just kind of a nicety.