Add key-mapping customization option

I found that I constantly hit `<Space>` by accident when marking visual
selections, which I do frequently. I tried to map the pipe2eval key to
something else but found it was impossible.

 - Add a default map key to continue current behaviour
 - Add a global variable to hold user-defined custom key for mapping
   - g:pipe2eval_map_key
 - Modify plugin/pipe2eval.vim to accept both custom key and default
 - Update the README.md to relect the change

I found that there was no `:help pipe2eval`(!).

 - Add doc/pipe2eval.txt