Modules path and highlight links are wrong

[quezak]

Hello! I just started using this repository to get syntax highlighting for ES6 features, and I found a couple of issues:

• The modules include path is syntax/vim-es6/modules/*.vim, but the modules are placed in syntax/modules/ directory.
• Before doing syntax highlight linking, modules check if exists("did_javascript_hilink"), but this variable isn't set anywhere.
• The modules use the HiLight function, which is defined in the main syntax file later, thus it's inaccessible by the modules.

I fixed these errors to make things work for me, I'll make a pull request in a minute :)