[R7RS] load has an optional second argument

[jpellegrini]

Hi @egallesio !

R7RS says that load should accept a second optional argument:

(load filename environment-specifier)

It would be a STklos module.

I looked into lib/load.stk, but I'm not yet familiar with that part of the code, so I didn't make a PR...

[egallesio]

It's already on my TODO list 😃 Don't worry, I'll do it (but probably for after 2.00, because the code about loading is too complicated and needs some rewriting).