aspell: optimization on spellers to improve speed (save state by buffer), add info "aspell_dict", add completion "aspell_dicts"

Two hashtables have been added to store the spellers currently used (in all
buffers) and info for each buffer: array with pointers to spellers and state of
aspell for this buffer.
Therefore the spellers are not destroyed and created again on each buffer switch,
or each time screen is refreshed when window is split.