Fix propagation of buffer-local variables
Fix propagation of buffer-local variables

Resolves #401.

Every time `(with-temp-buffer)` is used, any local variables of the
calling buffer are not propagated down.

One such variable is `intero-stack-executable`, which the user might
want to change locally (potentially, using .dir-locals.el). Since
processes are most often started in new, temporary, non-file-visiting
buffers, these new buffers need to see the value requested by the
user.

commercialhaskell/intero,commercialhaskell/intero,chrisdone/ghci-ng,chrisdone/ghci-ng,chrisdone/intero,commercialhaskell/intero,chrisdone/intero,chrisdone/intero