Retrieves a substring of this instance.
The substring starts at a specified character position and has a specified length.
|_string||String to extract from [String]|
|_startIndex||Index to start the substring extraction [Number]|
|_length||length of the extracted substring [Number](Optional) if is not set than from _startIndex to end|
String extracted [String]
_str = ["Fish frog cheese fromage", 5, 4] call CBA_fnc_substr; // => "frog"
joko // Jonas