Vimshell strategy

Strategy on top of [vimshell](https://github.com/Shougo/vimshell.vim).

Brings the following on the table (especially cool for GUI):

1. ANSI color support.
2. Allows using interactive commands (e.g. irb) and sending SIGINT (with `<C-c>`).
3. Shows command output while it's running.
4. A lot more that vimshell allows to do.

The following also concern me:

1. No pid (is this a problem?).
2. `dispatch#copen(0)` is called directly in `postexit` hook. I suppose it is not the best way to do that.
3. No support for background builds because hooks are not trigger for hidden vimshell buffer.
4. No `dispatch#vimshell#activate()` at the moment but I think it can be easily added, I just need to understand the expected behaviour.
5. Changes `updatetime` to low values while process is running. Vimshell adds autocmd on `CursorHold,CursorHoldI` to update output when vimshell buffer is not focused. Low `updatetime` works fine for me in GUI, but in Terminal this makes cursor blink like a crazy one. Any ideas on how to improve that?
6. Command exit code is not correct (it uses exit code from `tee`). I believe it is not really important and not all strategies implement that correctly but it can be fixed with a bit more bash.

I have implemented my own function to strip color codes from piped file with perl, but I know tmux strategy does the same, so it might make sense to share to code.