Add gpython to the benchmark

[ncw]

No description provided.

[d5]

Thanks for creating this PR! Do you mind posting an example output of this here? I'm curious what it's like compared to others.

[d5]

https://github.com/d5/tengo#benchmark

[ncw]

Sure

	fib(35)	fibt(35)	Type
Go	68ms	1ms	Go (native)
Tengo	5,261ms	1ms	VM on Go
Lua	1,720ms	1ms	Lua (native)
go-lua	6,240ms	1ms	Lua VM on Go
GopherLua	6,505ms	2ms	Lua VM on Go
Python	2,969ms	9ms	Python (native)
gpython	25,423ms	4ms	Python Interpreter on Go
starlark-go	20,703ms	2ms	Python-like Interpreter on Go
otto	130,110ms	10ms	JS Interpreter on Go
Anko	153,654ms	3ms	Interpreter on Go

These are on my Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz laptop.

So gpython makes a below average score, but not too bad!