Implement async invocation of rc in neovim

[sphaerophoria]

Vim hanging on heavy rc calls can be kind of irritating, so I've implemented an async implementation in neovim. In theory this could be extended to support dispatch or vim 8 job control as well.

A summary of the change is

• Create generic function to execute any function pipeline after an ExecuteRC \ This function takes a list of functions (with optional arguments) that all accept the output of RC
• Convert functions that call ExecuteRC to use the new pipeline function
• Create async version of ExecuteRC that sets up the handlers
• Smile

(i ran auto-indentation so there may be some interesting whitespace changes)

[lyuts]

Thank you for the change. Yeah auto indentation generated some noise in the diff, and would suggest not mixing refactoring/reformatting with the actual change by making it a separate commit, that way it is easier to get to the actual code change. I had to manually resolve some conflicts, so may I ask you to double check it, to make sure I didn't break it during this resolution. Thanks again.