Feature pipe stdout from execution cell

[mdtrooper]

I change a lot of code to add the pipe feature.

This example shows the new feature (and argument -i or --pipe):

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import iplantuml"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "TEMPLATE = \"\"\"\n",
    "@startuml\n",
    "\"{person1}\" -> \"{person2}\": \"{send}\"\n",
    "\"{person1}\" <- \"{person2}\": \"{receive}\"\n",
    "@enduml\n",
    "\"\"\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "%%plantuml --pipe\n",
    "\n",
    "print(TEMPLATE.format(person1='Me', person2='Trump',\n",
    "                     send='Give me a 1 billion dolars.',\n",
    "                     receive='I send you 10 billions dolars.'))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.6.8"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}

[mdtrooper]

The last ❌ in Travis is for it cannot download 3.3 python.

Added a example:
Untitled.ipynb.txt

Untitled.html.txt