I noticed that when I load a markdown file in vim, the `vim-json` file
automatically runs `set conceallevel=2`. There's a few configs which are
necessary to reproduce this:

1. vimrc should have `let g:markdown_fenced_languages = [ 'json', ... ]`
2. Install `elzr/vim-json` plugin
3. Load any markdown file

When these conditions are met, the markdown plugin will load the
`vim-json/syntax/json.vim` script. The problem is that this script does
something unusual by also loading the corresponding `ftplugin` script.
Since loading the `ftplugin` script is the root cause, the fix is to
stop loading this script. Because the `g:vim_json_syntax_conceal` plugin
might be uninitialized now, this guards the references with `exists()`
expressions. This is the same pattern as used in the builtin json syntax
plugin (`vim/runtime/syntax/json.vim`).

I tested this with:

* `set conceallevel=2 | let g:vim_json_syntax_conceal = 0`
* `set conceallevel=0 | let g:vim_json_syntax_conceal = 1`
* and also with default settings

Fixes elzr#104