Function: substring

--Usage--
> substring(some_string, begin: number, end: number)

Extract a subsection of a string.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:string]]		String to extract something from.
| @begin@	[[type:int]] (optional)	Index of first character to keep (the first index is 0).
		  			If ommitted, starts at the begining of the string.
| @end@		[[type:int]] (optional)	Index just beyond the last character to keep.
		  			If ommitted, runs to the end of the string.

--Examples--
> substring("abcdefg", end:2)          == "ab"
> substring("abcdefg", begin:2)        == "cdefg"
> substring("abcdefg", begin:2, end:4) == "cd"
> substring("abcdefg", begin:2, end:2) == ""
