Allow customizing individual concealing characters

The `g:javascript_conceal` boolean option is replaced by multiple `g:javascript_conceal_*` char options to allow customizing individual concealing characters. 

I only want to conceal the common `function` and `return` keywords with custom concealing chars instead of the default one, but it wasn't possible before since `g:javascript_conceal` is a wholesale. 

With this change, I can do so by adding the following in my `.vimrc`: 

```vim
let g:javascript_conceal_function   = "λ"
let g:javascript_conceal_return     = "↩"
```