Syntax highlighting fixes

- **Inconsistencies between Lua.tmLanguage and Lua.JSON-tmLanguage**

    Pull requests #7, #9, and #12 updated either Lua.tmLanguage or Lua.JSON-tmLanguage, but not both. I've updated both files with the changes, so that they are equivalent.

- **String escape enhancements**

   I've removed escape matching from long bracket strings, which is consistent with the Lua syntax. I've also added highlighting for numerical escapes (e.g. `\123`).