[Vim] Introduce s:try() and mappings

Flexibly define mappings to commands with fallbacks. Also
s:try_wincmd(), which is similar but for window commands.
[Vim] Install obvious-resize
[Vim] Install Rhubarb.vim

GitHub extensions to Fugitive.vim.
[Vim] Create map to :Start
[Vim] Add map for swapping single/double quotes
[Vim] Consolidate CtrlP user command for git repos