enhanced terminal tracking/Pid input

[jam1015]

Apologies in advance for a slightly monolithic pull request. I had submitted this pull request to the main vim-slime repo, and the feedback I got was to write an extension for vim-slime-ext-plugins. At the time I didn't have the bandwidth to go through the code and figure out how to do that. I saw your extension and the code you wrote gave me the base knowledge to figure out how slime-ext-plugins works. The two main features that I ad are

1. Enhanced tracking of terminals. Using autocommands in a file in the plugin directory`, PID and internal Neovim job ids are kept track of in a stack of dictionaries. Users are always prompted to use the terminal on top of the stack, and terminal info is popped off when the terminal is closed.

2. Being prompted with PID number to select a terminal rather than the Neovim job id. This is better because the PID is visible in the status line of the terminal.

I've written an excessively verbose README. This is because I always appreciate it when documentation is written at a level where a novice can understand.

I'm being slightly presumptuous here, and this would completely be based on your preference, but instead of completing this pull request I could make a pull request on the main vim -slime-ext-plugins repo to link to my fork.

[Klafyvel]

Hi, that's amazing, thank you. I'll have a more complete look tonight and most likely merge it.

[Klafyvel]

Sorry, I'm a bit late because my neovim configuration was utterly broken. Just tested your PR and I love it, it works like a charm.