Add support for automatically re-attaching to the runner pane

[crcunningham]

This might be very specific to my usage of this project but I often end up in a state with a detached runner when sending commands from another plugin using VtrSendCommandToRunner!. I thought I'd put this PR up in case this is a problem other people run in to as well.

With my usage of this plugin I want all of my commands to go to the same runner command. I created an option that makes that happen, g:VtrAutomaticReattachByName.

The changes are:

• Add config option: g: VtrAutomaticReattachByName which will automatically reattach to a pane with the named defined by g: VtrCreatedRunnerPaneName if it exists
• Add config option g: VtrCreatedRunnerPaneName which lets users configure the name used when creating panes
• Add a function to get a map of the pane names to pane ids
• Add an extra arg to AttachToSpecifiedPane which defines whether anything should be printed when the attach is successful
  • For the code I wrote it's not necessary or desired to have the status echo

[christoomey]

@crcunningham thanks for the updates here! I'll be honest, I don't find myself using the detach / re-attach flow much these days, but these changes seem useful for folks like yourself who do and otherwise scoped enough that I'm hopeful they won't cause any complexity elsewhere. Thanks for the PR!

[crcunningham]

Cool, I'll keep living on it and send any bug fixes your way if I find issues.