Multiple terminals

[GoldsteinE]

• Multiple terminals support
• Autostart terminal on ^S
• Connect buffer to terminal via :Iconn
  Closes Support for multiple terminals at once #8

[williamjameshandley]

This PR adds some pretty awesome functionality. There's plenty to discuss here, but I can confirm that it's already working better than vimteractive 1.7.

You should also add yourself to 'Authors' in README.rst (if you wish)

[williamjameshandley]

Once this PR is complete, and the documentation tidied, I think it would be appropriate to label this with the version number 2.0, in recognition of the substantial upgrade in usage and authorship.

[williamjameshandley]

@GoldsteinE : Has there been any progress on this? If not, I'm happy to finish off this PR, but would be good to know that you're not currently working on this before I do.

(I've been using your branch for the past two weeks and found it to be very effective)

[GoldsteinE]

I can finish this in the next month or you can do it.

[williamjameshandley]

OK. I think I've managed to implement the changes we discussed. The connections and terminal types are now controlled by buffer-specific variables.

I believe the functionality is nearly identical to the one originally implemented by @GoldsteinE, but would appreciate feedback as to whether there is anything remaining.

[GoldsteinE]

This PR seems ready. Is anything else needed here?

[williamjameshandley]

Many thanks @GoldsteinE. This has been a very successful and substantial contribution to vimteractive.