add custom surround inspired @isundaylee's PR

[t9md]

Inspired by and continuation of #1102

• New customSurroundPairs config is introduced, its value is JSON string with following key-value pair.
  • key: char to access
  • value: array [openText:string, closeText:string, addSpace:boolean]
  • addSpace is optional, it control padding space.
• Config can be edited in setting-view.
• These custom surround can be used in all surround operation shown below.
  • surround
  • delete-surround
  • change-surround
• JSON config is validated and user see warning dialog if it has error.

Configuration example

"vim-mode-plus":
  customSurroundPairs: '''
    {
      "p": ["<?php", "?>", true],
      "%": ["<%", "%>", true],
      "=": ["<%=", "%>", true],
      "s": ["\\"", "\\""]
    }
  '''

With the above example and y s is mapped to surround and c S is mapped to change-surround

• y s i w p:(surround inner-word p(PHP tag)).
• c S p %:(change p(PHP-tag) surround to %(Ruby-irb) tag.