Implement custom variable register api

[hrsh7th]

Fixes #93, #86

Overview

This PR aims to register a custom variable resolver that can realize some of the interpolation related functionality.
For example, we can register the below variable for vim interpolation.

function! s:EVAL_VIM(context) abort
  let l:script = join(map(copy(a:context.node.children), 'v:val.text()'), '')
  return eval(l:script)
endfunction
call vsnip#variable#register('EVAL_VIM', function('s:EVAL_VIM'))

  "changedtick": {
    "prefix": "var",
    "body": [
      "${EVAL_VIM:getbufvar('%', 'changedtick')}"
    ]
  },

Currently, custom variable context is the below structure.

let a:context = {
\   'node': s:Variable, " the resolving variable node
\   'before': string, " the forward snippet text for the resolving variable before sync
\   'after': string, " the backward snippet text for the resolving variable before sync
\ }

TODO

• Pass $TM_SELECTED_TEXT test case
• Docs
• Add some tests
• Fix potential bug of variable children

[hrsh7th]

It should be removed for built-in.

[hrsh7th]

It has to turn on as default.

[hrsh7th]

I want to provide synched text for the variable argument... but it's hard...

[hrsh7th]

Close for a moment.