Do concealing in syntax file only if enabled.

I installed [indentLine](https://github.com/Yggdroot/indentLine) plugin, and got issues like mentioned in issue https://github.com/elzr/vim-json/issues/23. 

Since setting 

`let g:vim_json_syntax_conceal = 0` and 
`let g:indentLine_noConcealCursor=""` or `let g:indentLine_noConcealCursor="nc"`

didn't work out properly, i noticed that when conceal is disabled manually (set to 0), syntax file still does concealing. Only the conceal cursor and level are not handled in that case. IndentLine sets conceallevel to 2, which automatically enables it for json, and i believe when someone disables the concealing manually expects not to have any of those functionality enabled, even when concealcursor/conceallevel is changed.

This is fixing the problem. I'm just not sure if it's good practice to use variables in syntax files.