Adds configurable key bindings using rc.

[jbnicolai]

See documentation

The appname is 'vimdebug' and key bindings are under vim.keys., e.g.: vim.keys.break.

For example, to configure the add-breakpoint key to

./lib/vim-inspector.js --vim.keys.break="C-b"

or a ~/.vimdebugrc containing:

{
  "vim": {
    "keys": {
      "break": "C-b"
    }
  }
  // vim: set ft=json:
}

Closes #13.

[jbnicolai]

Could definitely use some review feedback, as I'm still quite unfamiliar with the architecture.

// @sidorares