Add joinEnv and joinUrl string functions and 2 new system vars

[solvingj]

Add two new system variables which map directly to platform specific go constants:

• PATH_LIST_SEPARATOR -> os.PathListSeparator
• FILE_PATH_SEPARATOR -> os.PathSeparator

Add joinEnv string function which is sibling to joinPath:

• strings.Join(elem, string(os.PathListSeparator))

Add joinUrl string function which is sibling to joinPath:

• path.Join(elem...)

Add all to docs

closes #2406

[solvingj]

Up to @pd93 and @andreynering I suppose.

[solvingj]

For these variables, you need to define them in getSpecialVars inside compiler.go

Ah, of course, moved...

[vmaerten]

@solvingj the lint is failing. Do you still want to work on it?

[solvingj]

Yah I can fix it tomorrow. Just to confirm, are you sure you want to merge it?