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 docs(!).
* Add doc/pipe2eval.txt