Add eros-inspect-last-result

[isamert]

A feature I miss from CIDER is that cider-inspect-last-result. This
PR provides somewhat similar feature to that. CIDER has it's own very
powerful inspector but here I just used the pp-display-expression
function from pp.el.

When eros-inspect-last-result is called, it shows the result of last
eros-eval-last-sexp in a pretty printed format in a different buffer
named *eros-inspect*. If the *eros-inspect* buffer is already
created, every call to eros-eval-last-sexp updates the contents of
the *eros-inspect* buffer automatically.

I don't know if you want to include something like this (or even if
you are maintaining the project actively still) but I was going to
implement it for myself and thought I could open a PR.

[xiongtx]

Thanks!