feature proposal: terminal in split screen + REPL

Dear Zyedidia and Micro contributors,

as a long time terminal user i am very impressed with your work. This is the first project which can possibly replace Vim or Emacs for me. It is full-featured even without plugins, it use powerful editing paradigm and still very lightweight!

There is only one daily-use feature i still can't reproduce in Micro: terminal in split screen with ability to send lines/blocks/selection from editor to terminal.

This is very useful feature if you developing script in some interpreted language: python, bash, nodejs, etc..

Ideal behavior looks like this (i'll try to describe it in NeoVim commands because it already have this behavior out-of-the-box and you can easily try it to see how it works):

1. open main file with code, say, bash script
2. execute editor command: :split term://bash
3. switch back to editor window with Esc, C-w, C-w
4. navigate to some line with code
5. run copy-past command like this: "tyy<C-w>w"tpa<CR><C-\><C-n><C-w>pj - it can be mapped to hotkey. It will copy whole line, paste and execute into terminal and then navigate back to your editor window.

In short - just search for "NeoVim terminal"

It is very useful behavior to develope or debug simple scripts in terminal. It is also useful to quickly open terminal in split acreen, rename few files and close it.

Also this behavior is universal - you can work with any interpreted language: perl, python, ruby, bash, powershel, nodejs, whatever.

With this feature Micro will become powerful enough to replace Vim/Emacs/Neovim in lot of scenarios.

Start by reproducing the proposed :split term://bash workflow and compare it with the described NeoVim terminal behavior. The completed feature should provide a split terminal, allow lines, blocks, or selections to be sent from the editor, and support interactive REPL use across the mentioned interpreted languages.