add foreplay#output_in_buffer option

Provide option to send the results of evaluating code to a special
result buffer instead of just echoing it. Changes include:

- add plugin/foreplay/buffers.vim containing the functions to manage
  the special result buffer

- add s:clear_result_buffer and s:print_output to foreplay.vim

- modify foreplay#evalprint and s:output_response to use s:print_output
  instead of echo