Support of variables within more/all build system fields

[SaschaMzH]

There are a lot of possibilities to customize the build system with variables.
But I miss the possibility to use the variables also within the env arbitrary option.

For example:
The following sublime-project file will print PYTHONPATH=${project_path}/my_path instead of the replaced project path

{
    "folders":
    [
        {
            "name": "Env Examples",
            "path": "."
        }
    ],
    "build_systems":
    [
        {
            "name": "Echo",
            "env":{
                "PYTHONPATH":"${project_path}/my_path"
            },
            "working_dir":"${project_path}",
            "shell_cmd": "printenv"
        }
    ]
}

[FichteFoll]

I expanded this issue to cover more/all fields in build systems. It doesn't really make sense to limit expansion to only a few keys when it could find use in almost every field, so might as well apply it to all. If the literal ${something} is desired, then ST allows escaping dollar signs in its snippet syntax too.

By the way, since besides cmd and shell_cmd all relevant fields are expanded in Default/exec.py anyway, it would be as easy as adding a few calls to the API in that file. It could even be grouped cleverly and reduced to one call.

Example: sublime.expand_variables(working_dir, sublime.extract_variables())

Related: #259

[keith-hall]

also related: #918