Extracts the index-based substring from a string.
| _string | String to extract from [String] | 
| _startIndex | Index to start the substring extraction [Number] | 
| _endIndex | Index to end the substring extraction [Number] | 
String extracted [String]
_str = ["Fish frog cheese fromage", 5, 8] call CBA_fnc_substring; // => "frog"
jaynus, joko // Jonas